diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index bd0d419..2e8a944 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -539,30 +539,28 @@ protected function buildItems(array $tree) {
     $items = [];
 
     foreach ($tree as $data) {
-      $class = ['menu-item'];
-      // Generally we only deal with visible links, but just in case.
+      $element = [];
+
       if (!$data['link']['access']) {
         continue;
       }
       // Set a class for the <li>-tag. Since $data['below'] may contain local
-      // tasks, only set 'expanded' class if the link also has children within
+      // tasks, only set 'expanded' true if the link also has children within
       // the current book.
       if ($data['link']['has_children'] && $data['below']) {
-        $class[] = 'menu-item--expanded';
+        $element['is_expanded'] = TRUE;
       }
       elseif ($data['link']['has_children']) {
-        $class[] = 'menu-item--collapsed';
+        $element['is_collapsed'] = TRUE;
       }
 
-      // Set a class if the link is in the active trail.
+      // Set a helper variable to indicate whether link is in active trail.
       if ($data['link']['in_active_trail']) {
-        $class[] = 'menu-item--active-trail';
+        $element['in_active_trail'] = TRUE;
       }
 
       // Allow book-specific theme overrides.
-      $element = [];
       $element['attributes'] = new Attribute();
-      $element['attributes']['class'] = $class;
       $element['title'] = $data['link']['title'];
       $node = $this->entityManager->getStorage('node')->load($data['link']['nid']);
       $element['url'] = $node->urlInfo();
diff --git a/core/modules/toolbar/templates/menu--toolbar.html.twig b/core/modules/toolbar/templates/menu--toolbar.html.twig
index 4165072..e8e6b45 100644
--- a/core/modules/toolbar/templates/menu--toolbar.html.twig
+++ b/core/modules/toolbar/templates/menu--toolbar.html.twig
@@ -32,14 +32,14 @@
       <ul class="toolbar-menu">
     {% endif %}
       {% for item in items %}
-      {%
-        set classes = [
-          'menu-item',
-          item.is_expanded ? 'menu-item--expanded',
-          item.is_collapsed ? 'menu-item--collapsed',
-          item.in_active_trail ? 'menu-item--active-trail',
-        ]
-      %}
+        {%
+          set classes = [
+            'menu-item',
+            item.is_expanded ? 'menu-item--expanded',
+            item.is_collapsed ? 'menu-item--collapsed',
+            item.in_active_trail ? 'menu-item--active-trail',
+          ]
+        %}
         <li{{ item.attributes.addClass(classes) }}>
           {{ link(item.title, item.url) }}
           {% if item.below %}
diff --git a/core/themes/classy/templates/navigation/book-tree.html.twig b/core/themes/classy/templates/navigation/book-tree.html.twig
index 543696c..f6c7f02 100644
--- a/core/themes/classy/templates/navigation/book-tree.html.twig
+++ b/core/themes/classy/templates/navigation/book-tree.html.twig
@@ -30,7 +30,15 @@
       <ul class="menu">
     {% endif %}
       {% for item in items %}
-        <li{{ item.attributes }}>
+        {%
+          set classes = [
+            'menu-item',
+            item.is_expanded ? 'menu-item--expanded',
+            item.is_collapsed ? 'menu-item--collapsed',
+            item.in_active_trail ? 'menu-item--active-trail',
+          ]
+        %}
+        <li{{ item.attributes.addClass(classes) }}>
           {{ link(item.title, item.url) }}
           {% if item.below %}
             {{ book_tree.book_links(item.below, attributes, menu_level + 1) }}