diff --git a/modules/book/book.install b/modules/book/book.install
index 93c712b..5254369 100644
--- a/modules/book/book.install
+++ b/modules/book/book.install
@@ -18,6 +18,10 @@ function book_install() {
  * Implements hook_uninstall().
  */
 function book_uninstall() {
+  variable_del('book_allowed_types');
+  variable_del('book_child_type');
+  variable_del('book_block_mode');
+
   // Delete menu links.
   db_delete('menu_links')
     ->condition('module', 'book')