diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index bfe55881af..dfa0f90407 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -591,6 +591,13 @@ protected function buildItems(array $tree) {
         $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'];
@@ -1080,7 +1087,10 @@ protected function buildBookOutlineData(array $links, array $parents = [], $dept
    */
   protected function buildBookOutlineRecursive(&$links, $parents, $depth) {
     $tree = [];
+    $node = \Drupal::routeMatch()->getParameter('node');
     while ($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/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
index e55fe36525..e784d713a1 100644
--- a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
+++ b/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
@@ -140,6 +140,7 @@ public function build() {
           // 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/core/modules/book/templates/book-tree.html.twig
index bf7424f4ef..6e8196e972 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/core/themes/classy/templates/navigation/book-tree.html.twig
index 186a547e14..68043be52f 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) }}>