How do I access variables displayed in devel?
If you want to do any theming to node use node.tpl.php instead of page.tpl.php

in page it is not suggested. If you want to render a field in node.tpl just write the following

&lt;?php print render($content['menu_name'])?&gt;


or simply

&lt;?php print $node-&gt;book['menu_name']; ?&gt;