Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/book/templates/book-navigation.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to navigate books.
+ *
+ * Presented under nodes that are a part of book outlines.
+ *
+ * Available variables:
+ * - tree: The immediate children of the current node rendered as an unordered
+ *   list.
+ * - current_depth: Depth of the current node within the book outline. Provided
+ *   for context.
+ * - prev_url: URL to the previous node.
+ * - prev_title: Title of the previous node.
+ * - parent_url: URL to the parent node.
+ * - parent_title: Title of the parent node. Not printed by default. Provided
+ *   as an option.
+ * - next_url: URL to the next node.
+ * - next_title: Title of the next node.
+ * - has_links: Flags TRUE whenever the previous, parent or next data has a
+ *   value.
+ * - book_id: The book ID of the current outline being viewed. Same as the node
+ *   ID containing the entire outline. Provided for context.
+ * - book_url: The book/node URL of the current outline being viewed. Provided
+ *   as an option. Not used by default.
+ * - book_title: The book/node title of the current outline being viewed.
+ *
+ * @see template_preprocess_book_navigation()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if tree or has_links %}
+  <nav role="navigation" aria-labelledby="book-label-{{ book_id }}">
+    {{ tree }}
+    {% if has_links %}
+      <h2>{{ 'Book traversal links for'|t }} {{ book_title }}</h2>
+      <ul>
+      {% if prev_url %}
+        <li>
+          <a href="{{ prev_url }}" rel="prev" title="{{ 'Go to previous page'|t }}"><b>{{ '‹'|t }}</b> {{ prev_title }}</a>
+        </li>
+      {% endif %}
+      {% if parent_url %}
+        <li>
+          <a href="{{ parent_url }}" title="{{ 'Go to parent page'|t }}">{{ 'Up'|t }}</a>
+        </li>
+      {% endif %}
+      {% if next_url %}
+        <li>
+          <a href="{{ next_url }}" rel="next" title="{{ 'Go to next page'|t }}">{{ next_title }} <b>{{ '›'|t }}</b></a>
+        </li>
+      {% endif %}
+    </ul>
+    {% endif %}
+  </nav>
+{% endif %}