+
+        book_descendants = list(book.children.all())
+        while len(book_descendants) > 0:
+            child_book = book_descendants.pop(0)
+            for fragment in child_book.fragments.all():
+                fragment.tags = set(list(fragment.tags) + [book_tag])
+            book_descendants += list(child_book.children.all())
+