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
@@ -590,7 +590,7 @@
    *     are children of one of the parent link IDs in this list. If empty,
    *     the whole outline is built, unless 'only_active_trail' is TRUE.
    *   - active_trail: An array of node IDs, representing the currently active
-   *      book link.
+   *     book link.
    *   - only_active_trail: Whether to only return links that are in the active
    *     trail. This option is ignored if 'expanded' is non-empty.
    *   - min_depth: The minimum depth of book links in the resulting tree.
@@ -621,21 +621,21 @@
    *   The ID of the book that we are building the tree for.
    * @param array $parameters
    *   (optional) An associative array of build parameters. Possible keys:
-   *   - expanded: An array of parent link ids to return only book links that
+   *   - expanded: An array of parent link IDs to return only book links that
    *     are children of one of the parent link IDs in this list. If empty,
    *     the whole outline is built, unless 'only_active_trail' is TRUE.
-   *   - active_trail: An array of nids, representing the coordinates of the
+   *   - active_trail: An array of node IDs, representing the coordinates of the
    *     currently active book link.
    *   - only_active_trail: Whether to only return links that are in the active
-   *     trail. This option is ignored, if 'expanded' is non-empty.
+   *     trail. This option is ignored if 'expanded' is non-empty.
    *   - min_depth: The minimum depth of book links in the resulting tree.
-   *     Defaults to 1, which is the default to build a whole tree for a book.
+   *     Defaults to 1, which is to build the whole tree for the book.
    *   - max_depth: The maximum depth of book links in the resulting tree.
    *   - conditions: An associative array of custom database select query
    *     condition key/value pairs; see _menu_build_tree() for actual query.
    *
    * @return array
-   *   Tree of a book.
+   *   An array with links representing the tree structure of the book.
    *
    * @see menu_build_tree()
    */