Get book bid to display at search results page
The book id bid is nothing more then the node id nid of the parent. Therefor if you have the bid, you still need to extract the title from the parent node.

$search_hit = $variables['result']['node']-&gt;nid; // The node id of the search result item
$node = node_load($search_hit); // Load the whole node whose search result we got
$parent_id = $node-&gt;book['bid']; // Extract the parent from the found node

$node2 = node_load($parent_id); // Load the parent node.
echo $node2-&gt;title; // This is the parent title.