Processes variables for book-export-html.tpl.php.
$variables: An associative array containing the following keys:
function template_preprocess_book_export_html(&$variables) {
global $base_url, $language;
$variables['title'] = check_plain($variables['title']);
$variables['base_url'] = $base_url;
$variables['language'] = $language;
$variables['language_rtl'] = $language->direction == LANGUAGE_RTL;
$variables['head'] = drupal_get_html_head();
$variables['dir'] = $language->direction ? 'rtl' : 'ltr';
}