diff --git a/core/modules/book/src/BookManagerInterface.php b/core/modules/book/src/BookManagerInterface.php
index 72ca61e..f3e4a89 100644
--- a/core/modules/book/src/BookManagerInterface.php
+++ b/core/modules/book/src/BookManagerInterface.php
@@ -57,6 +57,14 @@ public function getActiveTrailIds($bid, $link);
   /**
    * Loads a single book entry.
    *
+   * The entries of a book entry is documented in
+   * \Drupal\book\BookOutlineStorageInterface::loadMultiple.
+   *
+   * if $translate is TRUE, it also checks access ('access' key) and
+   * loads the title from the node itself.
+   *
+   * On top of it, loadBookLinks
+   *
    * @param int $nid
    *   The node ID of the book.
    * @param bool $translate
@@ -70,6 +78,12 @@ public function loadBookLink($nid, $translate = TRUE);
   /**
    * Loads multiple book entries.
    *
+   * The entries of a book entry is documented in
+   * \Drupal\book\BookOutlineStorageInterface::loadMultiple.
+   *
+   * if $translate is TRUE, it also checks access ('access' key) and
+   * loads the title from the node itself.
+   *
    * @param int[] $nids
    *   An array of nids to load.
    *
diff --git a/core/modules/book/src/BookOutlineStorageInterface.php b/core/modules/book/src/BookOutlineStorageInterface.php
index e9b0e0e..9c94412 100644
--- a/core/modules/book/src/BookOutlineStorageInterface.php
+++ b/core/modules/book/src/BookOutlineStorageInterface.php
@@ -31,6 +31,14 @@ public function hasBooks();
   /**
    * Loads books.
    *
+   * Each book entry consists of the following keys:
+   *   - bid: The node ID of the main book.
+   *   - nid: The node ID of the book entry itself.
+   *   - pid: The parent node ID of the book.
+   *   - has_children: A boolean to indicate whether the book has children.
+   *   - weight: The weight of the book entry to order siblings.
+   *   - depth: The depth in the menu hierarchy the entry is placed into.
+   *
    * @param array $nids
    *   An array of node IDs.
    * @param bool $access