diff --git a/core/modules/book/book.admin.inc b/core/modules/book/book.admin.inc
index 7d9e88b..014b899 100644
--- a/core/modules/book/book.admin.inc
+++ b/core/modules/book/book.admin.inc
@@ -104,7 +104,7 @@ function book_admin_edit_submit($form, &$form_state) {
       if ($row['parent']['plid']['#default_value'] != $values['parent']['plid'] || $row['weight']['#default_value'] != $values['weight']) {
         $menu_link = entity_load('menu_link', $values['parent']['mlid']);
         $menu_link->weight = $values['weight'];
-        $menu_link->plid = $values['plid'];
+        $menu_link->plid = $values['parent']['plid'];
         $menu_link->save();
         $updated = TRUE;
       }
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index b7a3a18..09fdc3d 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -978,7 +978,8 @@ function book_preprocess_block(&$variables) {
  *
  * @param array $variables
  *   An associative array containing the following key:
- *   - book_menus
+ *   - book_menus: An associative array containing renderable menu links for all
+ *     book menus.
  */
 function template_preprocess_book_all_books_block(&$variables) {
   // Remove all non-renderable elements.
@@ -996,7 +997,8 @@ function template_preprocess_book_all_books_block(&$variables) {
  *
  * @param array $variables
  *   An associative array containing the following key:
- *   - book_link
+ *   - book_link: An associative array of book link properties.
+ *     Properties used: bid, link_title, depth, plid, mlid.
  */
 function template_preprocess_book_navigation(&$variables) {
   drupal_add_css(drupal_get_path('module', 'book') . '/book.theme.css');
@@ -1117,10 +1119,10 @@ function book_toc($bid, $depth_limit, $exclude = array()) {
  * Default template: book-export-html.twig.html
  *
  * @param array $variables
- *   An associative array containing the following keys:
- *   - title
- *   - contents
- *   - depth
+ *   An associative array containing:
+ *   - title: The title of the book.
+ *   - contents: Output of each book page.
+ *   - depth: The max depth of the book.
  */
 function template_preprocess_book_export_html(&$variables) {
   global $base_url;
@@ -1202,14 +1204,15 @@ function book_node_export(EntityInterface $node, $children = '') {
 }
 
 /**
- * Processes variables for single node export templates.
+ * Prepares variables for single node export templates.
  *
  * Default template: book-node-export-html.html.twig.
  *
  * @param array $variables
  *   An associative array containing the following keys:
- *   - node
- *   - children
+ *   - node: The node that will be output.
+ *   - children: All the rendered child nodes within the current node. Defaults
+ *     to an empty string.
  */
 function template_preprocess_book_node_export_html(&$variables) {
   $variables['depth'] = $variables['node']->book['depth'];
diff --git a/core/modules/book/lib/Drupal/book/Tests/BookTest.php b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
index 4601342..ae44640 100644
--- a/core/modules/book/lib/Drupal/book/Tests/BookTest.php
+++ b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
@@ -501,12 +501,12 @@ public function testBookOrdering() {
       "table[book-admin-{$node1->nid}][weight]" => 1,
       "table[book-admin-{$node2->nid}][weight]" => 2,
       // Put node 2 under node 1.
-      "table[book-admin-{$node2->nid}][plid]" => $plid,
+      "table[book-admin-{$node2->nid}][parent][plid]" => $plid,
     );
     $this->drupalPost(NULL, $edit, t('Save book pages'));
     // Verify weight was updated.
     $this->assertFieldByName("table[book-admin-{$node1->nid}][weight]", 1);
     $this->assertFieldByName("table[book-admin-{$node2->nid}][weight]", 2);
-    $this->assertFieldByName("table[book-admin-{$node2->nid}][plid]", $plid);
+    $this->assertFieldByName("table[book-admin-{$node2->nid}][parent][plid]", $plid);
   }
 }
diff --git a/core/modules/book/templates/book-navigation.html.twig b/core/modules/book/templates/book-navigation.html.twig
index 8c873f0..82e02fd 100644
--- a/core/modules/book/templates/book-navigation.html.twig
+++ b/core/modules/book/templates/book-navigation.html.twig
@@ -36,21 +36,21 @@
   <nav id="book-navigation-{{ book_id }}" class="book-navigation">
     {{ tree }}
     {% if has_links %}
-      <h2 class="element-invisible">{{ 'Book Navigation' | t }}</h2>
+      <h2 class="element-invisible">{{ 'Book Navigation'|t }}</h2>
       <ul class="book-pager">
       {% if prev_url %}
         <li class="previous">
-          <a href="{{ prev_url }}" rel="prev" title="{{ 'Go to previous page' | t }}"><b>{{ '‹' | t }}</b> {{ prev_title }}</a>
+          <a href="{{ prev_url }}" rel="prev" title="{{ 'Go to previous page'|t }}"><b>{{ '‹'|t }}</b> {{ prev_title }}</a>
         </li>
       {% endif %}
       {% if parent_url %}
         <li class="up">
-          <a href="{{ parent_url }}" title="{{ 'Go to parent page' | t }}">{{ 'Up' | t }}</a>
+          <a href="{{ parent_url }}" title="{{ 'Go to parent page'|t }}">{{ 'Up'|t }}</a>
         </li>
       {% endif %}
       {% if next_url %}
         <li class="next">
-          <a href="{{ next_url }}" rel="next" title="{{ 'Go to next page' | t }}">{{ next_title }} <b>{{ '›' | t }}</b></a>
+          <a href="{{ next_url }}" rel="next" title="{{ 'Go to next page'|t }}">{{ next_title }} <b>{{ '›'|t }}</b></a>
         </li>
       {% endif %}
     </ul>