--- /dev/null
+{#
+/**
+ * @file
+ * Default theme implementation to display a book tree.
+ *
+ * Returns HTML for a wrapper for a book sub-tree.
+ *
+ * Available variables:
+ * - items: A nested list of book items. Each book item contains:
+ * - attributes: HTML attributes for the book item.
+ * - below: The book item child items.
+ * - title: The book link title.
+ * - url: The book link URL, instance of \Drupal\Core\Url.
+ * - is_expanded: TRUE if the link has visible children within the current
+ * book tree.
+ * - 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.
+ *
+ * @ingroup themeable
+ */
+#}
+{% import _self as book_tree %}
+
+{#
+ We call a macro which calls itself to render the full tree.
+ @see http://twig.sensiolabs.org/doc/tags/macro.html
+#}
+{{ book_tree.book_links(items, attributes, 0) }}
+
+{% macro book_links(items, attributes, menu_level) %}
+ {% import _self as book_tree %}
+ {% if items %}
+ {% if menu_level == 0 %}
+ <ul{{ attributes }}>
+ {% else %}
+ <ul>
+ {% endif %}
+ {% for item in items %}
+ <li{{ item.attributes }}>
+ {{ link(item.title, item.url) }}
+ {% if item.below %}
+ {{ book_tree.book_links(item.below, attributes, menu_level + 1) }}
+ {% endif %}
+ </li>
+ {% endfor %}
+ </ul>
+ {% endif %}
+{% endmacro %}