I had to get the parent nid of a book child page for a website which already uses Drupal 6.

Seems that the book module logic has changed deeply between version 5. The new stack implementation of the parent-child relationships is pretty complex, at least for me. I have to say that I did not studied it a lot thought.

So.. this is the code to do that:

<?php

if(isset($node->book)) { // book page
      $parent = book_link_load($node->book['plid']);
      $pid = $parent['nid'];
}

?>

Hope this helps.