diff --git a/backend/internal/bookfile/cache.go b/backend/internal/bookfile/cache.go
new file mode 100644
index 0000000..7667a4c
--- /dev/null
+++ b/backend/internal/bookfile/cache.go
@@ -0,0 +1,37 @@
+package bookfile
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+func DirKey(id int64, hash string) string { return fmt.Sprintf("%d-%s", id, hash) }
+
+func CoverDir(cacheDir, key string) string { return filepath.Join(cacheDir, "covers", key) }
+func PagesDir(cacheDir, key string) string { return filepath.Join(cacheDir, "pages", key) }
+
+func SweepStale(cacheDir string, live map[string]bool) (int, error) {
+	removed := 0
+	for _, kind := range []string{"covers", "pages"} {
+		entries, err := os.ReadDir(filepath.Join(cacheDir, kind))
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue // 首次运行还没有该层目录,不算错
+			}
+			return removed, err
+		}
+		for _, e := range entries {
+			if !e.IsDir() || live[e.Name()] {
+				continue
+			}
+			if err := os.RemoveAll(filepath.Join(cacheDir, kind, e.Name())); err != nil {
+				return removed, err
+			}
+			removed++
+		}
+	}
+	return removed, nil
+}
diff --git a/backend/internal/bookfile/cache_test.go b/backend/internal/bookfile/cache_test.go
new file mode 100644
index 0000000..72b98bf
--- /dev/null
+++ b/backend/internal/bookfile/cache_test.go
@@ -0,0 +1,53 @@
+package bookfile
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func mk(t *testing.T, cacheDir, kind, key, file string) {
+	t.Helper()
+	d := filepath.Join(cacheDir, kind, key)
+	os.MkdirAll(d, 0o755)
+	os.WriteFile(filepath.Join(d, file), []byte("x"), 0o644)
+}
+
+func TestSweepStale(t *testing.T) {
+	dir := t.TempDir()
+	mk(t, dir, "covers", "1-aaa", "cover.jpg")
+	mk(t, dir, "covers", "2-bbb", "cover.png")
+	mk(t, dir, "pages", "1-aaa", "0.jpg")
+	mk(t, dir, "pages", "3-ccc", "0.jpg")
+	os.WriteFile(filepath.Join(dir, "covers", "stray.txt"), []byte("x"), 0o644) // 非目录,忽略
+
+	n, err := SweepStale(dir, map[string]bool{"1-aaa": true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 2 {
+		t.Fatalf("removed %d want 2", n)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "covers", "1-aaa")); err != nil {
+		t.Fatal("live dir removed")
+	}
+	if _, err := os.Stat(filepath.Join(dir, "pages", "1-aaa")); err != nil {
+		t.Fatal("live pages removed")
+	}
+	if _, err := os.Stat(filepath.Join(dir, "covers", "2-bbb")); !os.IsNotExist(err) {
+		t.Fatal("stale cover remains")
+	}
+	// cacheDir 不存在 = 首次运行,不算错
+	if n, err := SweepStale(filepath.Join(dir, "nope"), nil); n != 0 || err != nil {
+		t.Fatalf("%d %v", n, err)
+	}
+}
+
+func TestPaths(t *testing.T) {
+	if DirKey(12, "ab") != "12-ab" {
+		t.Fatal("DirKey")
+	}
+	if CoverDir("/c", "12-ab") != filepath.Join("/c", "covers", "12-ab") {
+		t.Fatal("CoverDir")
+	}
+}
diff --git a/backend/internal/store/store.go b/backend/internal/store/store.go
index 054ba7b..4b98dbe 100644
--- a/backend/internal/store/store.go
+++ b/backend/internal/store/store.go
@@ -262,6 +262,24 @@ func (s *Store) ListBooks(ctx context.Context, libID int64, q, prefix string, us
 	return out, rows.Err()
 }
 
+func (s *Store) BookHashes(ctx context.Context) (map[int64][2]int64, error) {
+	rows, err := s.P.Query(ctx, "SELECT id, file_size, mod_ts FROM books")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := map[int64][2]int64{}
+	for rows.Next() {
+		var id int64
+		var v [2]int64
+		if err := rows.Scan(&id, &v[0], &v[1]); err != nil {
+			return nil, err
+		}
+		out[id] = v
+	}
+	return out, rows.Err()
+}
+
 // ---------- progress ----------
 
 func (s *Store) UpsertProgress(ctx context.Context, userID, libID int64, bookPath string, locator []byte, percent float64) error {
diff --git a/backend/internal/store/store_test.go b/backend/internal/store/store_test.go
index c5633e3..b91dda5 100644
--- a/backend/internal/store/store_test.go
+++ b/backend/internal/store/store_test.go
@@ -77,6 +77,20 @@ func TestBooksDiffLifecycle(t *testing.T) {
 	}
 }
 
+func TestBookHashes(t *testing.T) {
+	s := setup(t)
+	ctx := context.Background()
+	libID, _ := s.CreateLibrary(ctx, "l", "/data/books/l")
+	bid, _ := s.InsertBook(ctx, libID, "a.cbz", "a", "cbz", 100, 2000, 1)
+	m, err := s.BookHashes(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v := m[bid]; v[0] != 100 || v[1] != 2000 {
+		t.Fatalf("bad hashes %+v", v)
+	}
+}
+
 func TestProgressUpsertAndJoin(t *testing.T) {
 	s := setup(t)
 	ctx := context.Background()