diff --git a/src/App.test.tsx b/src/App.test.tsx
index d689121..e623f96 100644
--- a/src/App.test.tsx
+++ b/src/App.test.tsx
@@ -169,6 +169,7 @@ describe('OneNoteApplication', () => {
       sourceSize: 1,
       entries: [],
       sections: [],
+      tree: { interpreted: false, nodes: [] },
       diagnostics: [],
     };
     render(
@@ -237,11 +238,36 @@ describe('OneNoteApplication', () => {
           ],
         },
       ],
+      tree: {
+        interpreted: true,
+        tocPath: 'Open Notebook.onetoc2',
+        nodes: [
+          {
+            kind: 'section-group',
+            id: 'Notes',
+            displayName: 'Notes',
+            children: [
+              {
+                kind: 'section',
+                id: 'readable',
+                sectionId: 'readable',
+                displayName: 'Readable',
+              },
+              {
+                kind: 'section',
+                id: 'broken',
+                sectionId: 'broken',
+                displayName: 'Broken',
+              },
+            ],
+          },
+        ],
+      },
       diagnostics: [
         {
           severity: 'warning',
-          code: 'TOC_NOT_INTERPRETED',
-          message: 'Package path order is temporary.',
+          code: 'TEST_PACKAGE_WARNING',
+          message: 'Example recoverable package warning.',
           recoverable: true,
         },
       ],
@@ -263,10 +289,12 @@ describe('OneNoteApplication', () => {
     expect(
       await screen.findByRole('heading', { name: 'Package summary' })
     ).toBeInTheDocument();
+    expect(screen.getByText('Notes')).toBeVisible();
+    expect(screen.getByText('OneNote notebook order')).toBeVisible();
     expect(
       screen.getByRole('button', { name: /Broken, failed/i })
     ).toBeVisible();
-    expect(screen.getByText('TOC_NOT_INTERPRETED')).toBeInTheDocument();
+    expect(screen.getByText('TEST_PACKAGE_WARNING')).toBeInTheDocument();
     expect(
       await within(
         screen.getByRole('article', { name: 'First meeting' })
diff --git a/src/App.tsx b/src/App.tsx
index 999b332..ed75957 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -12,7 +12,9 @@ import {
   SingleSectionNavigation,
 } from './components/SectionNavigation.js';
 import type {
+  OneNoteNotebookNodeDto,
   OneNotePackageDto,
+  OneNotePackagedSectionDto,
   OneNotePageDto,
   OneNoteSectionDto,
 } from './onenote/model/dto.js';
@@ -75,6 +77,32 @@ function uniqueDiagnostics(
   });
 }
 
+function sectionsInNotebookOrder(
+  notebook: OneNotePackageDto
+): OneNotePackagedSectionDto[] {
+  const sectionById = new Map(
+    notebook.sections.map((section) => [section.id, section] as const)
+  );
+  const ordered: OneNotePackagedSectionDto[] = [];
+  const seen = new Set<string>();
+  const visit = (nodes: OneNoteNotebookNodeDto[]) => {
+    for (const node of nodes) {
+      if (node.kind === 'section-group') {
+        visit(node.children);
+        continue;
+      }
+      const section = sectionById.get(node.sectionId);
+      if (section && !seen.has(section.id)) {
+        ordered.push(section);
+        seen.add(section.id);
+      }
+    }
+  };
+  visit(notebook.tree.nodes);
+  ordered.push(...notebook.sections.filter((section) => !seen.has(section.id)));
+  return ordered;
+}
+
 export interface OneNoteApplicationProps {
   createWorkerClient?: () => OneNoteWorkerClient;
 }
@@ -209,17 +237,18 @@ export function OneNoteApplication({
         return;
       }
 
+      const orderedSections = sectionsInNotebookOrder(response.notebook);
       const preferredSection =
-        response.notebook.sections.find(
+        orderedSections.find(
           (section) =>
             section.parseStatus === 'parsed' &&
             section.section !== undefined &&
             section.section.pages.length > 0
         ) ??
-        response.notebook.sections.find(
+        orderedSections.find(
           (section) => section.parseStatus === 'parsed' && section.section
         ) ??
-        response.notebook.sections[0];
+        orderedSections[0];
       setSelectedSectionId(preferredSection?.id);
       setState({
         phase: 'loaded',
@@ -341,6 +370,7 @@ export function OneNoteApplication({
               />
             ) : (
               <PackageNavigation
+                tree={state.source.notebook.tree}
                 sections={state.source.notebook.sections}
                 selectedSectionId={selectedSectionId}
                 selectedPageId={selectedPageId}
diff --git a/src/components/SectionNavigation.test.tsx b/src/components/SectionNavigation.test.tsx
new file mode 100644
index 0000000..563e964
--- /dev/null
+++ b/src/components/SectionNavigation.test.tsx
@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest';
+
+import type { OneNotePageSummaryDto } from '../onenote/model/dto.js';
+import { buildPageTree } from './page-tree.js';
+
+function page(id: string, level?: number): OneNotePageSummaryDto {
+  return { id, level, title: id, text: '', textPreview: '' };
+}
+
+describe('section navigation trees', () => {
+  it('turns authored page levels into parent/subpage relationships', () => {
+    expect(
+      buildPageTree([
+        page('root', 1),
+        page('child', 2),
+        page('grandchild', 3),
+        page('next-root', 1),
+        page('skipped-level', 3),
+      ])
+    ).toEqual([
+      {
+        page: page('root', 1),
+        children: [
+          {
+            page: page('child', 2),
+            children: [{ page: page('grandchild', 3), children: [] }],
+          },
+        ],
+      },
+      {
+        page: page('next-root', 1),
+        children: [{ page: page('skipped-level', 3), children: [] }],
+      },
+    ]);
+  });
+
+  it('treats absent and invalid levels as top-level pages', () => {
+    expect(
+      buildPageTree([page('unset'), page('zero', 0), page('nan', Number.NaN)])
+    ).toMatchObject([
+      { page: { id: 'unset' }, children: [] },
+      { page: { id: 'zero' }, children: [] },
+      { page: { id: 'nan' }, children: [] },
+    ]);
+  });
+});
diff --git a/src/components/SectionNavigation.tsx b/src/components/SectionNavigation.tsx
index 4ae670e..ef8cc46 100644
--- a/src/components/SectionNavigation.tsx
+++ b/src/components/SectionNavigation.tsx
@@ -1,40 +1,69 @@
 import type {
+  OneNoteNotebookNodeDto,
+  OneNoteNotebookTreeDto,
   OneNotePackagedSectionDto,
   OneNotePageSummaryDto,
   OneNoteSectionDto,
 } from '../onenote/model/dto.js';
+import { buildPageTree, type PageTreeNode } from './page-tree.js';
 
 interface PageListProps {
   pages: OneNotePageSummaryDto[];
   selectedPageId?: string;
   onSelectPage(pageId: string): void;
+  nested?: boolean;
 }
 
-function PageList({ pages, selectedPageId, onSelectPage }: PageListProps) {
+function PageTreeItems({
+  nodes,
+  selectedPageId,
+  onSelectPage,
+}: {
+  nodes: PageTreeNode[];
+  selectedPageId?: string;
+  onSelectPage(pageId: string): void;
+}) {
+  return nodes.map(({ page, children }) => (
+    <li key={page.id}>
+      <button
+        type="button"
+        className={page.id === selectedPageId ? 'is-selected' : undefined}
+        aria-current={page.id === selectedPageId ? 'page' : undefined}
+        onClick={() => onSelectPage(page.id)}
+      >
+        <strong>{page.title || 'Untitled page'}</strong>
+        {page.textPreview ? <span>{page.textPreview}</span> : null}
+      </button>
+      {children.length > 0 ? (
+        <ul>
+          <PageTreeItems
+            nodes={children}
+            selectedPageId={selectedPageId}
+            onSelectPage={onSelectPage}
+          />
+        </ul>
+      ) : null}
+    </li>
+  ));
+}
+
+function PageList({
+  pages,
+  selectedPageId,
+  onSelectPage,
+  nested = false,
+}: PageListProps) {
   if (pages.length === 0) {
     return <p className="navigation-empty">No pages were extracted.</p>;
   }
 
   return (
-    <ul className="page-list">
-      {pages.map((page) => (
-        <li
-          key={page.id}
-          style={{
-            paddingInlineStart: `${Math.min(page.level ?? 0, 8) * 0.7}rem`,
-          }}
-        >
-          <button
-            type="button"
-            className={page.id === selectedPageId ? 'is-selected' : undefined}
-            aria-current={page.id === selectedPageId ? 'page' : undefined}
-            onClick={() => onSelectPage(page.id)}
-          >
-            <strong>{page.title || 'Untitled page'}</strong>
-            {page.textPreview ? <span>{page.textPreview}</span> : null}
-          </button>
-        </li>
-      ))}
+    <ul className={`page-list${nested ? ' page-list--nested' : ''}`}>
+      <PageTreeItems
+        nodes={buildPageTree(pages)}
+        selectedPageId={selectedPageId}
+        onSelectPage={onSelectPage}
+      />
     </ul>
   );
 }
@@ -67,6 +96,7 @@ export function SingleSectionNavigation({
 }
 
 interface PackageNavigationProps {
+  tree: OneNoteNotebookTreeDto;
   sections: OneNotePackagedSectionDto[];
   selectedSectionId?: string;
   selectedPageId?: string;
@@ -74,60 +104,109 @@ interface PackageNavigationProps {
   onSelectPage(pageId: string): void;
 }
 
+function NotebookTreeItems({
+  nodes,
+  sections,
+  selectedSectionId,
+  selectedPageId,
+  onSelectSection,
+  onSelectPage,
+}: {
+  nodes: OneNoteNotebookNodeDto[];
+  sections: ReadonlyMap<string, OneNotePackagedSectionDto>;
+  selectedSectionId?: string;
+  selectedPageId?: string;
+  onSelectSection(sectionId: string): void;
+  onSelectPage(pageId: string): void;
+}) {
+  return nodes.map((node) => {
+    if (node.kind === 'section-group') {
+      return (
+        <li className="section-group" key={node.id}>
+          <details open>
+            <summary>{node.displayName}</summary>
+            <ul>
+              <NotebookTreeItems
+                nodes={node.children}
+                sections={sections}
+                selectedSectionId={selectedSectionId}
+                selectedPageId={selectedPageId}
+                onSelectSection={onSelectSection}
+                onSelectPage={onSelectPage}
+              />
+            </ul>
+          </details>
+        </li>
+      );
+    }
+
+    const section = sections.get(node.sectionId);
+    if (!section) return null;
+    const selected = section.id === selectedSectionId;
+    return (
+      <li className="notebook-section" key={node.id}>
+        <button
+          type="button"
+          aria-label={`${section.displayName}, ${section.parseStatus}`}
+          className={selected ? 'is-selected' : undefined}
+          aria-current={selected ? 'true' : undefined}
+          onClick={() => onSelectSection(section.id)}
+        >
+          <strong>{section.displayName}</strong>
+          <span className={`status status--${section.parseStatus}`}>
+            {section.parseStatus}
+          </span>
+        </button>
+        {selected && section.section ? (
+          <PageList
+            pages={section.section.pages}
+            selectedPageId={selectedPageId}
+            onSelectPage={onSelectPage}
+            nested
+          />
+        ) : selected ? (
+          <p className="navigation-empty">
+            This section did not produce a readable page list.
+          </p>
+        ) : null}
+      </li>
+    );
+  });
+}
+
 export function PackageNavigation({
+  tree,
   sections,
   selectedSectionId,
   selectedPageId,
   onSelectSection,
   onSelectPage,
 }: PackageNavigationProps) {
-  const selected = sections.find((section) => section.id === selectedSectionId);
+  const sectionById = new Map(
+    sections.map((section) => [section.id, section] as const)
+  );
 
   return (
     <nav className="source-navigation" aria-label="Notebook sections and pages">
       <div className="navigation-heading">
         <p className="eyebrow">Sections</p>
         <h2>Notebook contents</h2>
-        <p>Temporary normalized-path order</p>
+        <p>{tree.interpreted ? 'OneNote notebook order' : 'Package order'}</p>
       </div>
-      {sections.length === 0 ? (
+      {tree.nodes.length === 0 ? (
         <p className="navigation-empty">No section entries were found.</p>
       ) : (
-        <ul className="section-list">
-          {sections.map((section) => (
-            <li key={section.id}>
-              <button
-                type="button"
-                aria-label={`${section.displayName}, ${section.parseStatus}`}
-                className={
-                  section.id === selectedSectionId ? 'is-selected' : undefined
-                }
-                aria-current={
-                  section.id === selectedSectionId ? 'true' : undefined
-                }
-                onClick={() => onSelectSection(section.id)}
-              >
-                <strong>{section.displayName}</strong>
-                <span className={`status status--${section.parseStatus}`}>
-                  {section.parseStatus}
-                </span>
-              </button>
-            </li>
-          ))}
+        <ul className="notebook-tree section-list">
+          <NotebookTreeItems
+            nodes={tree.nodes}
+            sections={sectionById}
+            selectedSectionId={selectedSectionId}
+            selectedPageId={selectedPageId}
+            onSelectSection={onSelectSection}
+            onSelectPage={onSelectPage}
+          />
         </ul>
       )}
-
-      {selected?.section ? (
-        <PageList
-          pages={selected.section.pages}
-          selectedPageId={selectedPageId}
-          onSelectPage={onSelectPage}
-        />
-      ) : selected ? (
-        <p className="navigation-empty">
-          This section did not produce a readable page list.
-        </p>
-      ) : null}
     </nav>
   );
 }
diff --git a/src/components/page-tree.ts b/src/components/page-tree.ts
new file mode 100644
index 0000000..0246214
--- /dev/null
+++ b/src/components/page-tree.ts
@@ -0,0 +1,29 @@
+import type { OneNotePageSummaryDto } from '../onenote/model/dto.js';
+
+export interface PageTreeNode {
+  page: OneNotePageSummaryDto;
+  children: PageTreeNode[];
+}
+
+/** Convert OneNote's flat authored page-level sequence to a bounded tree. */
+export function buildPageTree(pages: OneNotePageSummaryDto[]): PageTreeNode[] {
+  const roots: PageTreeNode[] = [];
+  const ancestors: PageTreeNode[] = [];
+
+  for (const page of pages) {
+    const level = Number.isFinite(page.level) ? Math.trunc(page.level ?? 1) : 1;
+    const authoredDepth = Math.max(0, Math.min(level - 1, 8));
+    // Attach a skipped level to the deepest parent that actually exists.
+    const depth = Math.min(authoredDepth, ancestors.length);
+    const node: PageTreeNode = { page, children: [] };
+    if (depth === 0) {
+      roots.push(node);
+    } else {
+      ancestors[depth - 1]!.children.push(node);
+    }
+    ancestors[depth] = node;
+    ancestors.length = depth + 1;
+  }
+
+  return roots;
+}
diff --git a/src/onenote/model/dto.ts b/src/onenote/model/dto.ts
index 306bc96..3f732e1 100644
--- a/src/onenote/model/dto.ts
+++ b/src/onenote/model/dto.ts
@@ -49,12 +49,39 @@ export interface OneNotePackageEntryDto {
   parseStatus: 'not-requested' | 'parsed' | 'unsupported' | 'failed';
 }
 
+export type OneNoteNotebookNodeDto =
+  | {
+      kind: 'section';
+      id: string;
+      sectionId: string;
+      displayName: string;
+      orderingId?: number;
+      color?: number;
+    }
+  | {
+      kind: 'section-group';
+      id: string;
+      displayName: string;
+      orderingId?: number;
+      color?: number;
+      children: OneNoteNotebookNodeDto[];
+    };
+
+export interface OneNoteNotebookTreeDto {
+  /** True when the tree is rooted in a successfully parsed package TOC. */
+  interpreted: boolean;
+  tocPath?: string;
+  color?: number;
+  nodes: OneNoteNotebookNodeDto[];
+}
+
 export interface OneNotePackageDto {
   format: 'onepkg';
   sourceName: string;
   sourceSize: number;
   entries: OneNotePackageEntryDto[];
   sections: OneNotePackagedSectionDto[];
+  tree: OneNoteNotebookTreeDto;
   diagnostics: ParserDiagnostic[];
 }
 
diff --git a/src/onenote/onepkg/joplin-fixture.test.ts b/src/onenote/onepkg/joplin-fixture.test.ts
index f88b397..69f6713 100644
--- a/src/onenote/onepkg/joplin-fixture.test.ts
+++ b/src/onenote/onepkg/joplin-fixture.test.ts
@@ -84,6 +84,23 @@ describe('pinned Joplin .onepkg fixture', () => {
         (section) => section.parseStatus === 'not-requested'
       )
     ).toBe(true);
+    expect(opened.package.tree).toMatchObject({
+      interpreted: true,
+      tocPath: 'Open Notebook.onetoc2',
+      nodes: [
+        { kind: 'section', displayName: 'Tést!' },
+        { kind: 'section', displayName: 'Another section' },
+        {
+          kind: 'section-group',
+          displayName: 'Section group',
+          children: [
+            { kind: 'section', displayName: 'A' },
+            { kind: 'section', displayName: 'B' },
+          ],
+        },
+        { kind: 'section', displayName: '⅀⸨ Unicode ⸩' },
+      ],
+    });
   });
 
   it('opens and parses all five packaged sections end to end', async () => {
diff --git a/src/onenote/onepkg/package.ts b/src/onenote/onepkg/package.ts
index 3fc9bf1..f6fdac9 100644
--- a/src/onenote/onepkg/package.ts
+++ b/src/onenote/onepkg/package.ts
@@ -8,6 +8,7 @@ import type { ParserDiagnostic } from '../model/diagnostics.js';
 import { checkCancellation } from './cancellation.js';
 import { extractCabinet } from './cabinet.js';
 import { diagnosticFromUnknown, OnePkgError, warning } from './errors.js';
+import { buildOneNotePackageTree } from './toc-hierarchy.js';
 import type {
   CabOpenOptions,
   ExtractedCabFile,
@@ -162,6 +163,12 @@ export async function openOneNotePackage(
     });
   }
 
+  const treeResult = buildOneNotePackageTree(
+    extraction.extractedEntries,
+    packagedSections
+  );
+  diagnostics.push(...treeResult.diagnostics);
+
   return {
     package: {
       format: 'onepkg',
@@ -169,6 +176,7 @@ export async function openOneNotePackage(
       sourceSize: bytes.byteLength,
       entries: packageEntries,
       sections: packagedSections,
+      tree: treeResult.tree,
       diagnostics,
     },
     extractedEntries: extraction.extractedEntries,
diff --git a/src/onenote/onepkg/toc-hierarchy.test.ts b/src/onenote/onepkg/toc-hierarchy.test.ts
new file mode 100644
index 0000000..78c64a7
--- /dev/null
+++ b/src/onenote/onepkg/toc-hierarchy.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+
+import { resolveTocChildPath } from './toc-hierarchy.js';
+
+describe('ONEPKG TOC child paths', () => {
+  it('joins a single NFC-normalized child name within its section group', () => {
+    expect(resolveTocChildPath('Projects', 'Cafe\u0301.one')).toBe(
+      'Projects/Café.one'
+    );
+  });
+
+  it.each(['../Secret.one', '..', 'nested/Secret.one', 'C:\\Secret.one'])(
+    'rejects unsafe FolderChildFilename %s',
+    (filename) => {
+      expect(() => resolveTocChildPath('Projects', filename)).toThrow(
+        /Unsafe FolderChildFilename/u
+      );
+    }
+  );
+});
diff --git a/src/onenote/onepkg/toc-hierarchy.ts b/src/onenote/onepkg/toc-hierarchy.ts
new file mode 100644
index 0000000..deed07b
--- /dev/null
+++ b/src/onenote/onepkg/toc-hierarchy.ts
@@ -0,0 +1,476 @@
+import { BinaryReader } from '../binary/BinaryReader.js';
+import { readGuid } from '../binary/guid.js';
+import type {
+  OneNoteNotebookNodeDto,
+  OneNoteNotebookTreeDto,
+  OneNotePackagedSectionDto,
+} from '../model/dto.js';
+import type { ParserDiagnostic } from '../model/diagnostics.js';
+import {
+  ONE_SECTION_FILE_TYPE,
+  detectOneNoteFormat,
+} from '../parser/detect-format.js';
+import { OneNoteParserError } from '../parser/error.js';
+import {
+  parseOneNoteTableOfContents,
+  type OneNoteTableOfContents,
+  type OneNoteTableOfContentsEntry,
+} from '../parser/parse-toc.js';
+import { fail, OnePkgError, warning } from './errors.js';
+import type { ExtractedCabFile } from './types.js';
+import { DEFAULT_ONEPKG_LIMITS } from './types.js';
+
+interface ParsedTocEntry {
+  extracted: ExtractedCabFile;
+  toc: OneNoteTableOfContents;
+  directory: string;
+}
+
+export interface OneNotePackageTreeResult {
+  tree: OneNoteNotebookTreeDto;
+  diagnostics: ParserDiagnostic[];
+}
+
+/**
+ * Interpret package TOCs and join them to extracted sections without ever
+ * materializing an archive path on the host filesystem.
+ */
+export function buildOneNotePackageTree(
+  extractedEntries: ExtractedCabFile[],
+  sections: OneNotePackagedSectionDto[]
+): OneNotePackageTreeResult {
+  const diagnostics: ParserDiagnostic[] = [];
+  const parsedTocs: ParsedTocEntry[] = [];
+
+  for (const extracted of extractedEntries) {
+    if (!extracted.normalizedPath.toLowerCase().endsWith('.onetoc2')) continue;
+    try {
+      const toc = parseOneNoteTableOfContents(extracted.bytes, {
+        sourceName: extracted.normalizedPath,
+      });
+      parsedTocs.push({
+        extracted,
+        toc,
+        directory: parentPath(extracted.normalizedPath),
+      });
+      for (const diagnostic of toc.diagnostics) {
+        if (diagnostics.length >= DEFAULT_ONEPKG_LIMITS.maxFiles) break;
+        diagnostics.push({
+          ...diagnostic,
+          structure: diagnostic.structure
+            ? `${extracted.normalizedPath} · ${diagnostic.structure}`
+            : extracted.normalizedPath,
+        });
+      }
+    } catch (error) {
+      diagnostics.push(tocFailureDiagnostic(error, extracted.normalizedPath));
+    }
+  }
+
+  const root = selectRootToc(parsedTocs, diagnostics);
+  const sectionByPath = new Map(
+    sections.map((section) => [pathKey(section.path), section])
+  );
+  const extractedSectionByPath = new Map(
+    extractedEntries
+      .filter((entry) => entry.normalizedPath.toLowerCase().endsWith('.one'))
+      .map((entry) => [pathKey(entry.normalizedPath), entry])
+  );
+  const sectionIdentityByPath = new Map<string, string>();
+  for (const [key, entry] of extractedSectionByPath) {
+    const identity = readSectionIdentity(entry.bytes);
+    if (identity) sectionIdentityByPath.set(key, identity);
+  }
+  const tocByDirectory = new Map<string, ParsedTocEntry>();
+  for (const parsed of parsedTocs) {
+    const key = pathKey(parsed.directory);
+    if (tocByDirectory.has(key)) {
+      diagnostics.push(
+        warning(
+          'onetoc2-directory-ambiguous',
+          `More than one readable .onetoc2 exists in ${parsed.directory || 'the package root'}; the first one is used.`,
+          { structure: parsed.directory || 'ONEPKG root' }
+        )
+      );
+      continue;
+    }
+    tocByDirectory.set(key, parsed);
+  }
+
+  const usedSectionIds = new Set<string>();
+  let nodes: OneNoteNotebookNodeDto[] = [];
+  if (root) {
+    nodes = buildTocNodes({
+      parsed: root,
+      tocByDirectory,
+      sectionByPath,
+      sectionIdentityByPath,
+      sections,
+      usedSectionIds,
+      diagnostics,
+      activeTocs: new Set(),
+      usedTocs: new Set(),
+      depth: 0,
+    });
+  }
+
+  const unlisted = sections
+    .filter(
+      (section) =>
+        !usedSectionIds.has(section.id) && !isRecycleBinPath(section.path)
+    )
+    .sort((left, right) => left.path.localeCompare(right.path));
+  if (unlisted.length > 0 && root) {
+    diagnostics.push(
+      warning(
+        'onetoc2-unlisted-sections',
+        `${unlisted.length} package section${unlisted.length === 1 ? '' : 's'} were not reachable from the parsed TOC and were appended to navigation.`,
+        { structure: root.extracted.normalizedPath }
+      )
+    );
+  }
+  nodes.push(...unlisted.map(fallbackSectionNode));
+
+  return {
+    tree: {
+      interpreted: root !== undefined,
+      tocPath: root?.extracted.normalizedPath,
+      color: root?.toc.color,
+      nodes,
+    },
+    diagnostics,
+  };
+}
+
+interface BuildTocNodesContext {
+  parsed: ParsedTocEntry;
+  tocByDirectory: ReadonlyMap<string, ParsedTocEntry>;
+  sectionByPath: ReadonlyMap<string, OneNotePackagedSectionDto>;
+  sectionIdentityByPath: ReadonlyMap<string, string>;
+  sections: OneNotePackagedSectionDto[];
+  usedSectionIds: Set<string>;
+  diagnostics: ParserDiagnostic[];
+  activeTocs: Set<string>;
+  usedTocs: Set<string>;
+  depth: number;
+}
+
+function buildTocNodes(
+  context: BuildTocNodesContext
+): OneNoteNotebookNodeDto[] {
+  if (context.depth > DEFAULT_ONEPKG_LIMITS.maxPathDepth) {
+    context.diagnostics.push(
+      warning(
+        'onetoc2-tree-depth-limit',
+        'Notebook section-group nesting exceeds the package path-depth limit.',
+        { structure: context.parsed.extracted.normalizedPath }
+      )
+    );
+    return [];
+  }
+  const tocKey = pathKey(context.parsed.extracted.normalizedPath);
+  if (context.activeTocs.has(tocKey)) {
+    context.diagnostics.push(
+      warning(
+        'onetoc2-tree-cycle',
+        'A cycle between notebook table-of-contents files was ignored.',
+        { structure: context.parsed.extracted.normalizedPath }
+      )
+    );
+    return [];
+  }
+  if (context.usedTocs.has(tocKey)) {
+    context.diagnostics.push(
+      warning(
+        'onetoc2-tree-duplicate',
+        'A repeated notebook section-group reference was ignored.',
+        { structure: context.parsed.extracted.normalizedPath }
+      )
+    );
+    return [];
+  }
+
+  context.activeTocs.add(tocKey);
+  context.usedTocs.add(tocKey);
+  try {
+    const result: OneNoteNotebookNodeDto[] = [];
+    for (const entry of context.parsed.toc.entries) {
+      if (isRecycleBinName(entry.filename)) continue;
+      try {
+        const childPath = resolveTocChildPath(
+          context.parsed.directory,
+          entry.filename
+        );
+        const section = resolveSection(context, childPath, entry);
+        if (section) {
+          if (context.usedSectionIds.has(section.id)) {
+            context.diagnostics.push(
+              warning(
+                'onetoc2-section-duplicate',
+                `Repeated TOC reference to ${entry.filename} was ignored.`,
+                { structure: context.parsed.extracted.normalizedPath }
+              )
+            );
+            continue;
+          }
+          context.usedSectionIds.add(section.id);
+          result.push({
+            kind: 'section',
+            id: section.id,
+            sectionId: section.id,
+            displayName: section.displayName,
+            orderingId: entry.orderingId,
+            color: entry.color,
+          });
+          continue;
+        }
+
+        const childToc = context.tocByDirectory.get(pathKey(childPath));
+        if (childToc) {
+          const childTocKey = pathKey(childToc.extracted.normalizedPath);
+          if (context.activeTocs.has(childTocKey)) {
+            context.diagnostics.push(
+              warning(
+                'onetoc2-tree-cycle',
+                'A cycle between notebook table-of-contents files was ignored.',
+                { structure: childToc.extracted.normalizedPath }
+              )
+            );
+            continue;
+          }
+          if (context.usedTocs.has(childTocKey)) {
+            context.diagnostics.push(
+              warning(
+                'onetoc2-tree-duplicate',
+                'A repeated notebook section-group reference was ignored.',
+                { structure: childToc.extracted.normalizedPath }
+              )
+            );
+            continue;
+          }
+          result.push({
+            kind: 'section-group',
+            id: childPath,
+            displayName: leafName(childPath),
+            orderingId: entry.orderingId,
+            color: entry.color,
+            children: buildTocNodes({
+              ...context,
+              parsed: childToc,
+              depth: context.depth + 1,
+            }),
+          });
+          continue;
+        }
+
+        context.diagnostics.push(
+          warning(
+            'onetoc2-entry-unresolved',
+            `TOC entry ${entry.filename} has no matching section or section group in the package.`,
+            { structure: context.parsed.extracted.normalizedPath }
+          )
+        );
+      } catch (error) {
+        context.diagnostics.push(
+          tocChildFailureDiagnostic(
+            error,
+            context.parsed.extracted.normalizedPath,
+            entry.filename
+          )
+        );
+      }
+    }
+    return result;
+  } finally {
+    context.activeTocs.delete(tocKey);
+  }
+}
+
+function resolveSection(
+  context: BuildTocNodesContext,
+  childPath: string,
+  tocEntry: OneNoteTableOfContentsEntry
+): OneNotePackagedSectionDto | undefined {
+  const expectedKey = pathKey(childPath);
+  const exact = context.sectionByPath.get(expectedKey);
+  if (exact) {
+    const identity = context.sectionIdentityByPath.get(expectedKey);
+    if (identity && identity !== tocEntry.fileIdentity) {
+      context.diagnostics.push(
+        warning(
+          'onetoc2-section-identity-mismatch',
+          `TOC identity for ${tocEntry.filename} does not match the section header; the package filename takes precedence.`,
+          { structure: exact.path }
+        )
+      );
+    }
+    return exact;
+  }
+
+  const directory = parentPath(childPath);
+  const identityMatches = context.sections.filter(
+    (section) =>
+      pathKey(parentPath(section.path)) === pathKey(directory) &&
+      context.sectionIdentityByPath.get(pathKey(section.path)) ===
+        tocEntry.fileIdentity
+  );
+  if (identityMatches.length === 1) {
+    const match = identityMatches[0]!;
+    context.diagnostics.push(
+      warning(
+        'onetoc2-section-filename-mismatch',
+        `TOC filename ${tocEntry.filename} was resolved to package section ${leafName(match.path)} by FileIdentityGuid.`,
+        { structure: context.parsed.extracted.normalizedPath }
+      )
+    );
+    return match;
+  }
+  return undefined;
+}
+
+function selectRootToc(
+  parsedTocs: ParsedTocEntry[],
+  diagnostics: ParserDiagnostic[]
+): ParsedTocEntry | undefined {
+  if (parsedTocs.length === 0) return undefined;
+  const sorted = [...parsedTocs].sort((left, right) => {
+    const depth = pathDepth(left.directory) - pathDepth(right.directory);
+    return (
+      depth ||
+      left.extracted.normalizedPath.localeCompare(
+        right.extracted.normalizedPath
+      )
+    );
+  });
+  const minimumDepth = pathDepth(sorted[0]!.directory);
+  const candidates = sorted.filter(
+    (candidate) => pathDepth(candidate.directory) === minimumDepth
+  );
+  if (candidates.length > 1) {
+    diagnostics.push(
+      warning(
+        'onetoc2-root-ambiguous',
+        'More than one top-level table of contents was found; the first safe package path is used.',
+        { structure: 'ONEPKG table of contents' }
+      )
+    );
+  }
+  return candidates[0];
+}
+
+export function resolveTocChildPath(
+  directory: string,
+  filename: string
+): string {
+  if (
+    filename.length === 0 ||
+    filename === '.' ||
+    filename === '..' ||
+    /[\\/:]/u.test(filename) ||
+    [...filename].some((character) => {
+      const codePoint = character.codePointAt(0)!;
+      return codePoint <= 0x1f || codePoint === 0x7f;
+    })
+  ) {
+    fail(
+      'onetoc2-child-path-unsafe',
+      `Unsafe FolderChildFilename value: ${filename}`,
+      { structure: 'FolderChildFilename' }
+    );
+  }
+  const normalized = filename.normalize('NFC');
+  const path = directory ? `${directory}/${normalized}` : normalized;
+  if (
+    path.length > DEFAULT_ONEPKG_LIMITS.maxPathCharacters ||
+    pathDepth(path) > DEFAULT_ONEPKG_LIMITS.maxPathDepth
+  ) {
+    fail(
+      'onetoc2-child-path-limit',
+      'FolderChildFilename exceeds package path limits.',
+      { structure: 'FolderChildFilename' }
+    );
+  }
+  return path;
+}
+
+function readSectionIdentity(bytes: Uint8Array): string | undefined {
+  const detected = detectOneNoteFormat(bytes);
+  if (
+    detected.fileType !== ONE_SECTION_FILE_TYPE ||
+    (detected.kind !== 'desktop-one' && detected.kind !== 'fsshttp-one') ||
+    bytes.byteLength < 32
+  ) {
+    return undefined;
+  }
+  return readGuid(new BinaryReader(bytes, 16, 16, 'OneStore Header.guidFile'));
+}
+
+function fallbackSectionNode(
+  section: OneNotePackagedSectionDto
+): OneNoteNotebookNodeDto {
+  return {
+    kind: 'section',
+    id: section.id,
+    sectionId: section.id,
+    displayName: section.displayName,
+  };
+}
+
+function tocFailureDiagnostic(error: unknown, path: string): ParserDiagnostic {
+  return {
+    severity: 'error',
+    code: 'onetoc2-parse-failed',
+    message: `Could not interpret ${path}${error instanceof Error ? `: ${error.message}` : '.'}`,
+    offset: error instanceof OneNoteParserError ? error.offset : undefined,
+    structure:
+      error instanceof OneNoteParserError && error.structure
+        ? `${path} · ${error.structure}`
+        : path,
+    recoverable: true,
+  };
+}
+
+function tocChildFailureDiagnostic(
+  error: unknown,
+  tocPath: string,
+  filename: string
+): ParserDiagnostic {
+  return {
+    severity: 'warning',
+    code:
+      error instanceof OnePkgError
+        ? error.diagnostic.code
+        : 'onetoc2-entry-failed',
+    message:
+      error instanceof Error
+        ? error.message
+        : `TOC entry ${filename} could not be resolved.`,
+    structure: `${tocPath} · FolderChildFilename`,
+    recoverable: true,
+  };
+}
+
+function parentPath(path: string): string {
+  const slash = path.lastIndexOf('/');
+  return slash < 0 ? '' : path.slice(0, slash);
+}
+
+function leafName(path: string): string {
+  return path.slice(path.lastIndexOf('/') + 1);
+}
+
+function pathDepth(path: string): number {
+  return path ? path.split('/').length : 0;
+}
+
+function pathKey(path: string): string {
+  return path.normalize('NFC').toLowerCase();
+}
+
+function isRecycleBinName(name: string): boolean {
+  return name.toLowerCase() === 'onenote_recyclebin';
+}
+
+function isRecycleBinPath(path: string): boolean {
+  return path.split('/').some(isRecycleBinName);
+}
diff --git a/src/onenote/parser/parse-toc.ts b/src/onenote/parser/parse-toc.ts
index a7d15bf..23f34d8 100644
--- a/src/onenote/parser/parse-toc.ts
+++ b/src/onenote/parser/parse-toc.ts
@@ -103,7 +103,7 @@ export function parseOneNoteTableOfContents(
   return {
     sourceName: options.sourceName ?? 'Open Notebook.onetoc2',
     fileIdentity: store.fileIdentity,
-    color: optionalU32(root, TOC_PROPERTY.color),
+    color: optionalColor(root),
     enableHistory: optionalBool(root, TOC_PROPERTY.enableHistory),
     entries,
     diagnostics,
@@ -151,7 +151,7 @@ function parseChildren(
             filename: decodeFolderChildFilename(filename),
             fileIdentity: requiredGuid(child, TOC_PROPERTY.fileIdentity),
             orderingId: requiredU32(child, TOC_PROPERTY.orderingId),
-            color: optionalU32(child, TOC_PROPERTY.color),
+            color: optionalColor(child),
           });
         } else {
           result.push(...parseChildren(context, child, depth + 1));
@@ -293,6 +293,11 @@ function optionalU32(
   return value.value;
 }
 
+function optionalColor(object: StoreObject): number | undefined {
+  const color = optionalU32(object, TOC_PROPERTY.color);
+  return color === 0xffff_ffff ? undefined : color;
+}
+
 function optionalBool(
   object: StoreObject,
   propertyId: number
diff --git a/src/styles.css b/src/styles.css
index ad22066..4c87814 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -190,6 +190,7 @@ p {
 
 .page-list,
 .section-list,
+.notebook-tree ul,
 .diagnostic-list {
   margin: 0;
   padding: 0;
@@ -201,7 +202,7 @@ p {
 }
 
 .page-list button,
-.section-list button {
+.notebook-section > button {
   display: grid;
   gap: 0.25rem;
   width: 100%;
@@ -223,7 +224,7 @@ p {
 }
 
 .page-list button.is-selected,
-.section-list button.is-selected {
+.notebook-section > button.is-selected {
   border-color: #af8bc6;
   color: #3e145c;
   background: #eee3f5;
@@ -233,15 +234,39 @@ p {
   padding: 0.55rem 0.55rem 0;
 }
 
-.section-list button {
+.notebook-section > button {
   grid-template-columns: minmax(0, 1fr) auto;
   align-items: center;
 }
 
-.section-list + .page-list {
-  margin: 0.55rem;
-  border-top: 1px solid #ded5e3;
-  padding-top: 0.85rem;
+.notebook-tree ul {
+  margin-inline-start: 0.7rem;
+  border-inline-start: 1px solid #ded5e3;
+  padding-inline-start: 0.45rem;
+}
+
+.section-group summary {
+  cursor: pointer;
+  padding: 0.62rem 0.45rem;
+  color: #574b5b;
+  font-size: 0.86rem;
+  font-weight: 700;
+}
+
+.notebook-section > .page-list {
+  margin: 0 0 0.4rem 0.7rem;
+  border-inline-start: 1px solid #ded5e3;
+  padding: 0.25rem 0 0.2rem 0.45rem;
+}
+
+.page-list ul {
+  margin: 0;
+  border: 0;
+  padding-inline-start: 0.7rem;
+}
+
+.page-list--nested button {
+  padding-block: 0.55rem;
 }
 
 .navigation-empty,
@@ -582,7 +607,7 @@ p {
   .drop-zone--active,
   button:hover:not(:disabled),
   .page-list button.is-selected,
-  .section-list button.is-selected {
+  .notebook-section > button.is-selected {
     color: #f6effa;
     background: #3c2b48;
   }
diff --git a/src/worker/parser-adapter.test.ts b/src/worker/parser-adapter.test.ts
index 4a0e2f3..0a7a6b3 100644
--- a/src/worker/parser-adapter.test.ts
+++ b/src/worker/parser-adapter.test.ts
@@ -43,9 +43,20 @@ describe('worker parser adapter', () => {
         (section) => section.parseStatus === 'parsed'
       )
     ).toBe(true);
-    expect(
-      result.notebook.diagnostics.map((diagnostic) => diagnostic.code)
-    ).toContain('PACKAGE_TOC_ORDER_NOT_INTERPRETED');
+    expect(result.notebook.tree.interpreted).toBe(true);
+    expect(result.notebook.tree.nodes).toMatchObject([
+      { kind: 'section', displayName: 'Tést!' },
+      { kind: 'section', displayName: 'Another section' },
+      {
+        kind: 'section-group',
+        displayName: 'Section group',
+        children: [
+          { kind: 'section', displayName: 'A' },
+          { kind: 'section', displayName: 'B' },
+        ],
+      },
+      { kind: 'section', displayName: '⅀⸨ Unicode ⸩' },
+    ]);
 
     for (const packagedSection of result.notebook.sections) {
       for (const page of packagedSection.section?.pages ?? []) {
diff --git a/src/worker/parser-adapter.ts b/src/worker/parser-adapter.ts
index a2a9b51..b90d491 100644
--- a/src/worker/parser-adapter.ts
+++ b/src/worker/parser-adapter.ts
@@ -83,39 +83,22 @@ export async function openPackageForWorker(
   });
 
   const pages = new Map<string, OneNotePageDto>();
-  const sections = [...result.package.sections]
-    .sort((left, right) => left.path.localeCompare(right.path))
-    .map((packagedSection) => {
-      if (!packagedSection.section) return packagedSection;
-      const section = {
-        ...packagedSection.section,
-        pages: packagedSection.section.pages.map((page) => {
-          pages.set(workerPageKey(page.id, packagedSection.id), pageDto(page));
-          return page;
-        }),
-      };
-      return { ...packagedSection, section };
-    });
+  const sections = result.package.sections.map((packagedSection) => {
+    if (!packagedSection.section) return packagedSection;
+    const section = {
+      ...packagedSection.section,
+      pages: packagedSection.section.pages.map((page) => {
+        pages.set(workerPageKey(page.id, packagedSection.id), pageDto(page));
+        return page;
+      }),
+    };
+    return { ...packagedSection, section };
+  });
 
   return {
     notebook: {
       ...result.package,
       sections,
-      diagnostics: [
-        ...result.package.diagnostics,
-        ...(sections.length > 0
-          ? [
-              {
-                severity: 'warning' as const,
-                code: 'PACKAGE_TOC_ORDER_NOT_INTERPRETED',
-                message:
-                  'OneNote notebook hierarchy and TOC ordering are not interpreted yet; sections are shown in normalized package-path order.',
-                structure: 'OneNote package section navigation',
-                recoverable: true,
-              },
-            ]
-          : []),
-      ],
     },
     pages,
   };