Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/modules/book/src/BookExport.php

@@ -65,12 +65,12 @@ class BookExport {
    * @param \Drupal\node\NodeInterface $node
    *   The node to export.
    *
-   * @throws \Exception
-   *   Thrown when the node was not attached to a book.
-   *
    * @return array
    *   A render array representing the HTML for a node and its children in the
    *   book hierarchy.
+   *
+   * @throws \Exception
+   *   Thrown when the node was not attached to a book.
    */
   public function bookExportHtml(NodeInterface $node) {
     if (!isset($node->book)) {

core/modules/book/src/BookManager.php

@@ -488,7 +488,6 @@ class BookManager implements BookManagerInterface {
    * {@inheritdoc}
    */
   public function getActiveTrailIds($bid, $link) {
-    $nid = isset($link['nid']) ? $link['nid'] : 0;
     // The tree is for a single item, so we need to match the values in its
     // p columns and 0 (the top level) with the plid values of other links.
     $active_trail = array(0);

core/modules/book/src/Plugin/Block/BookNavigationBlock.php

@@ -9,6 +9,7 @@ namespace Drupal\book\Plugin\Block;
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\book\BookManagerInterface;
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -182,12 +183,7 @@ class BookNavigationBlock extends BlockBase implements ContainerFactoryPluginInt
    * {@inheritdoc}
    */
   public function getCacheContexts() {
-    // The "Book navigation" block must be cached per role and book navigation
-    // context.
-    return [
-      'user.roles',
-      'route.book_navigation',
-    ];
+    return Cache::mergeContexts(parent::getCacheContexts(), ['route.book_navigation']);
   }
 
   /**

core/modules/book/src/ProxyClass/BookUninstallValidator.php

@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains Drupal\book\ProxyClass\BookUninstallValidator.
+ * Contains \Drupal\book\ProxyClass\BookUninstallValidator.
  */
 
 /**