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

core/modules/book/templates/book-all-books-block.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for rendering book outlines within a block.
+ *
+ * This template is used only when the block is configured to "show block on all
+ * pages", which presents multiple independent books on all pages.
+ *
+ * Available variables:
+ * - book_menus: Book outlines.
+ *   - id: The parent book ID.
+ *   - title: The parent book title.
+ *   - menu: The top-level book links.
+ *
+ * @see template_preprocess_book_all_books_block()
+ *
+ * @ingroup themeable
+ */
+#}
+{% for book in book_menus %}
+  <nav role="navigation" aria-label="{% trans %}Book outline for {{ book.title }}{% endtrans %}">
+    {{ book.menu }}
+  </nav>
+{% endfor %}