diff -u b/core/modules/book/book.module b/core/modules/book/book.module
--- b/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -227,9 +227,8 @@
  * Implements hook_entity_extra_field_info().
  */
 function book_entity_extra_field_info() {
-  $return = array();
+  $fields = [];
   $allowed_types = \Drupal::config('book.settings')->get('allowed_types');
-
   if ($allowed_types) {
     foreach ($allowed_types as $note_type) {
       $fields['node'][$note_type]['display']['book_navigation'] = [
@@ -239,8 +238,6 @@
       ];
     }
   }
-
-
   return $fields;
 }