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
@@ -253,7 +253,7 @@
    *   A data structure representing the tree as returned from buildBookOutlineData.
    *
    * @return array
-   *   A structured array to be rendered by renderer.
+   *   A structured renderable array.
    *
    * @todo This was copied from menu_tree_output() but with some changes that
    *   may be obsolete. Attempt to resolve the differences.