diff --git a/src/editor/positions.ts b/src/editor/positions.ts
index c804ce0..2fbbd7d 100644
--- a/src/editor/positions.ts
+++ b/src/editor/positions.ts
@@ -25,3 +25,23 @@ export function savePosition(bookId: string, chapterId: string, position: Chapte
   (all[bookId] ||= {})[chapterId] = position;
   localStorage.setItem(KEY, JSON.stringify(all));
 }
+
+const ACTIVE_KEY = "margin-active-chapter";
+
+function readActive(): Record<string, string> {
+  try {
+    return JSON.parse(localStorage.getItem(ACTIVE_KEY) || "{}") as Record<string, string>;
+  } catch {
+    return {};
+  }
+}
+
+export function loadActiveChapter(bookId: string): string | null {
+  return readActive()[bookId] ?? null;
+}
+
+export function saveActiveChapter(bookId: string, chapterId: string): void {
+  const all = readActive();
+  all[bookId] = chapterId;
+  localStorage.setItem(ACTIVE_KEY, JSON.stringify(all));
+}
diff --git a/src/store/useBook.ts b/src/store/useBook.ts
index c356ba5..8b7f2b0 100644
--- a/src/store/useBook.ts
+++ b/src/store/useBook.ts
@@ -12,6 +12,7 @@ import {
   createPage,
   normalizeBook,
 } from "../model/book";
+import { loadActiveChapter, saveActiveChapter } from "../editor/positions";
 
 export const COVER_ID = "__cover__";
 
@@ -50,7 +51,10 @@ export const useBook = create<BookState>((set) => ({
   setNotice: (message) => set({ notice: message }),
   openBook: (book) => {
     const normalized = normalizeBook(book);
-    set({ book: normalized, activeChapterId: normalized.chapters[0]?.id ?? "", dirty: false });
+    const saved = loadActiveChapter(normalized.id);
+    const valid = saved === COVER_ID || normalized.chapters.some((c) => c.id === saved);
+    const activeChapterId = valid ? (saved as string) : normalized.chapters[0]?.id ?? "";
+    set({ book: normalized, activeChapterId, dirty: false });
   },
   closeBook: () => set({ book: null, activeChapterId: "", dirty: false }),
   setActiveChapter: (id) => set({ activeChapterId: id }),
@@ -132,3 +136,11 @@ export const useBook = create<BookState>((set) => ({
     set((s) => (s.book ? { dirty: true, book: { ...s.book, cover: { ...s.book.cover, ...patch } } } : {})),
   markSaved: () => set({ dirty: false }),
 }));
+
+let lastActive = "";
+useBook.subscribe((s) => {
+  if (s.book && s.activeChapterId && s.activeChapterId !== lastActive) {
+    lastActive = s.activeChapterId;
+    saveActiveChapter(s.book.id, s.activeChapterId);
+  }
+});