diff --git a/core/modules/book/src/BookManager.php b/modules/book/src/BookManager.php
index b0cb5976..6243d148 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -619,6 +619,13 @@ class BookManager implements BookManagerInterface {
         $element['in_active_trail'] = TRUE;
       }

+      // Set a helper variable to indicate whether the link belongs to
+      // the current page.
+      $element['is_active'] = FALSE;
+      if ($data['link']['is_active']) {
+        $element['is_active'] = TRUE;
+      }
+
       // Allow book-specific theme overrides.
       $element['attributes'] = new Attribute();
       $element['title'] = $data['link']['title'];
@@ -1103,7 +1110,14 @@ class BookManager implements BookManagerInterface {
    */
   protected function buildBookOutlineRecursive(&$links, $parents, $depth) {
     $tree = [];
+    $node = \Drupal::routeMatch()->getParameter('node');
+    // The instanceof check doesn't filter out NULLs.
+    if ($node != NULL && !($node instanceof NodeInterface)) {
+      $node = $this->entityTypeManager->getStorage('node')->load($node);
+    }
     while ($item = array_pop($links)) {
+      // Check if item belongs to the current page.
+      $item['is_active'] = ($node && $node->id() == $item['nid']) ? TRUE : FALSE;
       // We need to determine if we're on the path to root so we can later build
       // the correct active trail.
       $item['in_active_trail'] = in_array($item['nid'], $parents);
diff --git a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php b/modules/book/src/Plugin/Block/BookNavigationBlock.php
index ed261dca..2df85731 100644
--- a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
+++ b/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
@@ -169,6 +169,7 @@ class BookNavigationBlock extends BlockBase implements ContainerFactoryPluginInt
           // Since we know we will only display a link to the top node, there
           // is no reason to run an additional menu tree query for each book.
           $book['in_active_trail'] = FALSE;
+          $book['is_active'] = FALSE;
           // Check whether user can access the book link.
           $book_node = $this->nodeStorage->load($book['nid']);
           $book['access'] = $book_node->access('view');
diff --git a/core/modules/book/templates/book-tree.html.twig b/modules/book/templates/book-tree.html.twig
index 7a818389..f3f203dc 100644
--- a/core/modules/book/templates/book-tree.html.twig
+++ b/core/modules/book/templates/book-tree.html.twig
@@ -16,6 +16,7 @@
  *   - is_collapsed: TRUE if the link has children within the current book tree
  *     that are not currently visible.
  *   - in_active_trail: TRUE if the link is in the active trail.
+ *   - is_active: TRUE if the link is for the current page.
  *
  * @ingroup themeable
  */
diff --git a/core/themes/classy/templates/navigation/book-tree.html.twig b/themes/classy/templates/navigation/book-tree.html.twig
index 94fac559..070e47b6 100644
--- a/core/themes/classy/templates/navigation/book-tree.html.twig
+++ b/core/themes/classy/templates/navigation/book-tree.html.twig
@@ -16,6 +16,7 @@
  *   - is_collapsed: TRUE if the link has children within the current book tree
  *     that are not currently visible.
  *   - in_active_trail: TRUE if the link is in the active trail.
+ *   - is_active: TRUE if the link is for the current page.
  */
 #}
 {% import _self as book_tree %}
@@ -41,6 +42,7 @@
           item.is_expanded ? 'menu-item--expanded',
           item.is_collapsed ? 'menu-item--collapsed',
           item.in_active_trail ? 'menu-item--active-trail',
+          item.is_active ? 'menu-item--active',
         ]
       %}
       <li{{ item.attributes.addClass(classes) }}>