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
@@ -62,9 +62,9 @@
     'book_navigation' => array(
       'variables' => array('book_link' => NULL),
     ),
-    'book_tree' => [
-      'variables' => ['items' => [], 'attributes' => []],
-    ],
+    'book_tree' => array(
+      'variables' => array('items' => array(), 'attributes' => array()),
+    ),
     'book_export_html' => array(
       'variables' => array('title' => NULL, 'contents' => NULL, 'depth' => NULL),
     ),
diff -u b/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
--- b/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -501,7 +501,7 @@
   /**
    * {@inheritdoc}
    */
-  public function build(array $tree) {
+  public function bookTreeOutput(array $tree) {
     $items = $this->buildItems($tree);
 
     $build = [];
@@ -525,7 +525,7 @@
   /**
    * Builds the #items property for a book tree's renderable array.
    *
-   * Helper function for ::build().
+   * Helper function for ::bookTreeOutput().
    *
    * @param array $tree
    *   A data structure representing the tree.
diff -u b/core/modules/book/src/BookManagerInterface.php b/core/modules/book/src/BookManagerInterface.php
--- b/core/modules/book/src/BookManagerInterface.php
+++ b/core/modules/book/src/BookManagerInterface.php
@@ -257,7 +257,7 @@
    *
    * @see \Drupal\Core\Menu\MenuLinkTree::build
    */
-  public function build(array $tree);
+  public function bookTreeOutput(array $tree);
 
   /**
    * Checks access and performs dynamic operations for each link in the tree.
reverted:
--- b/core/modules/book/src/BookOutline.php
+++ a/core/modules/book/src/BookOutline.php
@@ -126,7 +126,7 @@
     }
 
     if ($children) {
+      $elements = $this->bookManager->bookTreeOutput($children);
-      $elements = $this->bookManager->build($children);
       return drupal_render($elements);
     }
     return '';
reverted:
--- b/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
+++ a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
@@ -137,7 +137,7 @@
           // If the current page is a node associated with a book, the menu
           // needs to be retrieved.
           $data = $this->bookManager->bookTreeAllData($node->book['bid'], $node->book);
+          $book_menus[$book_id] = $this->bookManager->bookTreeOutput($data);
-          $book_menus[$book_id] = $this->bookManager->build($data);
         }
         else {
           // Since we know we will only display a link to the top node, there
@@ -147,7 +147,7 @@
           $book_node = $this->nodeStorage->load($book['nid']);
           $book['access'] = $book_node->access('view');
           $pseudo_tree[0]['link'] = $book;
+          $book_menus[$book_id] = $this->bookManager->bookTreeOutput($pseudo_tree);
-          $book_menus[$book_id] = $this->bookManager->build($pseudo_tree);
         }
         $book_menus[$book_id] += array(
           '#book_title' => $book['title'],
@@ -169,7 +169,7 @@
         $tree = $this->bookManager->bookTreeAllData($node->book['bid'], $node->book);
         // There should only be one element at the top level.
         $data = array_shift($tree);
+        $below = $this->bookManager->bookTreeOutput($data['below']);
-        $below = $this->bookManager->build($data['below']);
         if (!empty($below)) {
           return $below;
         }