commit 6b3c1dd28b268326becc30a1da3ee615a0917cf8
Author: Joel Pittet <joel@pittet.ca>
Date:   Sat May 24 16:20:19 2014 -0700

    book isn't using page it has contents variable

diff --git a/core/modules/book/templates/book-export-html.html.twig b/core/modules/book/templates/book-export-html.html.twig
index 7bbc4ab..cbeb3b3 100644
--- a/core/modules/book/templates/book-export-html.html.twig
+++ b/core/modules/book/templates/book-export-html.html.twig
@@ -39,7 +39,7 @@
   {% for i in 1..depth-1 if depth > 1 %}
     <div class="section-{{ i }}">
   {% endfor %}
-  {{ page.content }}
+  {{ contents }}
   {% for i in 1..depth-1 if depth > 1 %}
     </div>
   {% endfor %}