diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index c034ed6..5c93d73 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -335,18 +335,29 @@ function book_node_prepare_form(NodeInterface $node, $operation, FormStateInterf
 }
 
 /**
- * Implements hook_form_FORM_ID_alter() for node_delete_confirm().
+ * Implements hook_form_alter().
  *
  * Alters the confirm form for a single node deletion.
- *
- * @see node_delete_confirm()
  */
-function book_form_node_delete_confirm_alter(&$form, FormStateInterface $form_state) {
-  $node = Node::load($form['nid']['#value']);
+function book_form_alter(&$form, FormStateInterface $form_state, $form_id) {
+  $types = \Drupal::config('book.settings')->get('allowed_types');
+  $node_delete_form = FALSE;
+  foreach ($types as $type) {
+    if ($form_id == 'node_' . $type . '_delete_form') {
+      $node_delete_form = TRUE;
+      break;
+    }
+  }
+  if (!$node_delete_form) {
+    // Not a node delete form for book nodes.
+    return;
+  }
+
+  $node = $form_state->getFormObject()->getEntity();
 
   if (isset($node->book) && $node->book['has_children']) {
     $form['book_warning'] = array(
-      '#markup' => '<p>' . t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.', array('%title' => $node->label())) . '</p>',
+      '#markup' => '<p class="messages messages--warning">' . t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.', array('%title' => $node->label())) . '</p>',
       '#weight' => -10,
     );
   }
diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index 3e977b1..118b108 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -438,6 +438,7 @@ public function deleteFromBook($nid) {
       $result = $this->bookOutlineStorage->loadBookChildren($nid);
       $children = $this->entityManager->getStorage('node')->loadMultiple(array_keys($result));
       foreach ($children as $child) {
+        $child->book['bid'] = $child->id();
         $this->updateOutline($child);
       }
     }
diff --git a/core/modules/book/src/Tests/BookTest.php b/core/modules/book/src/Tests/BookTest.php
index 60e89ab..5195b9a 100644
--- a/core/modules/book/src/Tests/BookTest.php
+++ b/core/modules/book/src/Tests/BookTest.php
@@ -460,7 +460,7 @@ function testBookDelete() {
      $nodes = $this->createBook();
      $this->drupalLogin($this->adminUser);
      $this->drupalGet('node/' . $this->book->id() . '/delete');
-     $this->assertText(t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.', array('%title' => $this->book->label())));
+     $this->assertRaw(t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.', ['%title' => $this->book->label()]));
      // Delete parent, and visit a child page.
      $this->drupalPostForm('node/' . $this->book->id() . '/delete', [], t('Delete'));
      $this->drupalGet('node/' . $nodes[0]->id());