Form validation handler for book_admin_settings().
book_admin_settings_submit()
function book_admin_settings_validate($form, &$form_state) {
$child_type = $form_state['values']['book_child_type'];
if (empty($form_state['values']['book_allowed_types'][$child_type])) {
form_set_error('book_child_type', t('The content type for the %add-child link must be one of those selected as an allowed book outline type.', array(
'%add-child' => t('Add child page'),
)));
}
}