diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 09fdc3d..a735066 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -970,7 +970,7 @@ function book_preprocess_block(&$variables) {
 /**
  * Prepares variables for book listing block templates.
  *
- * Default template: book-all-books-block.twig.html.
+ * Default template: book-all-books-block.html.twig.
  *
  * All non-renderable elements are removed so that the template has full access
  * to the structured data but can also simply iterate over all elements and
@@ -993,7 +993,7 @@ function template_preprocess_book_all_books_block(&$variables) {
 /**
  * Prepares variables for book navigation templates.
  *
- * Default template: book-navigation.twig.html.
+ * Default template: book-navigation.html.twig.
  *
  * @param array $variables
  *   An associative array containing the following key:
@@ -1116,7 +1116,7 @@ function book_toc($bid, $depth_limit, $exclude = array()) {
 /**
  * Prepares variables for book export templates.
  *
- * Default template: book-export-html.twig.html
+ * Default template: book-export-html.html.twig
  *
  * @param array $variables
  *   An associative array containing:
diff --git a/core/modules/book/lib/Drupal/book/Tests/BookTest.php b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
index ae44640..4601342 100644
--- a/core/modules/book/lib/Drupal/book/Tests/BookTest.php
+++ b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
@@ -501,12 +501,12 @@ public function testBookOrdering() {
       "table[book-admin-{$node1->nid}][weight]" => 1,
       "table[book-admin-{$node2->nid}][weight]" => 2,
       // Put node 2 under node 1.
-      "table[book-admin-{$node2->nid}][parent][plid]" => $plid,
+      "table[book-admin-{$node2->nid}][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}][parent][plid]", $plid);
+    $this->assertFieldByName("table[book-admin-{$node2->nid}][plid]", $plid);
   }
 }
diff --git a/core/modules/book/templates/book-all-books-block.html.twig b/core/modules/book/templates/book-all-books-block.html.twig
index 241594d..518c1e1 100644
--- a/core/modules/book/templates/book-all-books-block.html.twig
+++ b/core/modules/book/templates/book-all-books-block.html.twig
@@ -8,8 +8,8 @@
  *
  * Available variables:
  * - book_menus: Book outlines.
- *   - book_id: the parent book ID.
- *   - menu: an array of top-level book links.
+ *   - book_id: The parent book ID.
+ *   - menu: The top-level book links.
  *
  * @see template_preprocess()
  * @see template_preprocess_book_all_books_block()
diff --git a/core/modules/book/templates/book-export-html.html.twig b/core/modules/book/templates/book-export-html.html.twig
index 1177f4c..e9d4e8c 100644
--- a/core/modules/book/templates/book-export-html.html.twig
+++ b/core/modules/book/templates/book-export-html.html.twig
@@ -7,8 +7,9 @@
  * - title: Top level node title.
  * - head: Header tags.
  * - language: Language object.
- * - language_rtl: TRUE or FALSE depending on right to left language scripts.
- * - base_url: URL to home page.
+ * - language_rtl: A flag indicating whether the current display language is a
+ *   right to left language.
+ * - base_url: URL to the home page.
  * - contents: Nodes within the current outline rendered through
  *   book-node-export-html.html.twig.
  *
@@ -31,14 +32,13 @@
   </head>
   <body>
     {#
-      The given node is /embedded to its absolute depth in a top level
-      section/. For example, a child node with depth 2 in the hierarchy is
-      contained in (otherwise empty) div elements corresponding to
-      depth 0 and depth 1. This is intended to support WYSIWYG output - e.g.,
-      level 3 sections always look like level 3 sections, no matter their
-      depth relative to the node selected to be exported as printer-friendly
-      HTML.
-     #}
+    The given node is embedded to its absolute depth in a top level section. For
+    example, a child node with depth 2 in the hierarchy is contained in
+    (otherwise empty) div elements corresponding to depth 0 and depth 1. This is
+    intended to support WYSIWYG output - e.g., level 3 sections always look like
+    level 3 sections, no matter their depth relative to the node selected to be
+    exported as printer-friendly HTML.
+    #}
 
   {% for i in range(1, depth-1) %}
     <div class="section-{{ i }}">
diff --git a/core/modules/book/templates/book-node-export-html.html.twig b/core/modules/book/templates/book-node-export-html.html.twig
index f8da328..7168de6 100644
--- a/core/modules/book/templates/book-node-export-html.html.twig
+++ b/core/modules/book/templates/book-node-export-html.html.twig
@@ -4,7 +4,7 @@
  * Default theme implementation for a single node in a printer-friendly outline.
  *
  * Available variables:
- * - node: Fully loaded node entity.
+ * - node: Fully loaded node.
  * - depth: Depth of the current node inside the outline.
  * - title: Node title.
  * - content: Node content.