Implements hook_node_presave().
function book_node_presave(Node $node) {
// Always save a revision for non-administrators.
if (!empty($node->book['bid']) && !user_access('administer nodes')) {
$node
->setNewRevision();
}
// Make sure a new node gets a new menu link.
if (empty($node->nid)) {
$node->book['mlid'] = NULL;
}
}