Processes variables for book-node-export-html.tpl.php.
$variables: An associative array containing the following keys:
function template_preprocess_book_node_export_html(&$variables) {
$variables['depth'] = $variables['node']->book['depth'];
$variables['title'] = check_plain($variables['node']->title);
$variables['content'] = $variables['node']->rendered;
}