commit 48714fe3801e3274455dcef669c161813360ec75
Author: Joel Pittet <joel@pittet.ca>
Date:   Wed May 27 22:51:04 2015 -0700

    left a level

diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index a0cb290..4015c4b 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -568,7 +568,7 @@ protected function buildItems(array $tree) {
       $element['url'] = $node->urlInfo();
       $element['localized_options'] = !empty($data['link']['localized_options']) ? $data['link']['localized_options'] : [];
       $element['localized_options']['set_active_class'] = TRUE;
-      $element['below'] = $data['below'] ? $this->build($data['below'], $level + 1) : [];
+      $element['below'] = $data['below'] ? $this->buildItems($data['below']) : [];
       $element['original_link'] = $data['link'];
       // Index using the link's unique nid.
       $items[$data['link']['nid']] = $element;