// does not work, in book_nodeapi:

    case 'delete':
      if (isset($node->book)) {
        drupal_delete_add_callback( array('book_delete_post' => array($node->book),));
        drupal_delete_add_query('DELETE FROM {book} WHERE mlid = %d', $node->book['mlid']);
      $form['node_'. $node->nid]['book_warning'] = array(
        '#value' => '<p>'. t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.' .'</p>', array('%title' => $node->title)),
        '#weight' => -10,
      ); 
      drupal_delete_add_form_elements($form);

      }

//works:

function book_delete_pre($type, $id, $package_info) {

  $form = array();

  if ($type == 'node' && is_numeric($id)) {
    $node = node_load($id);
    if (isset($node->book) && $node->book['has_children']) {
      $form['node_'. $node->nid]['book_warning'] = array(
        '#value' => '<p>'. t('%title is part of a book outline, and has associated child pages. If you proceed with deletion, the child pages will be relocated automatically.' .'</p>', array('%title' => $node->title)),
        '#weight' => -10,
      );
    }
  }
  return $form;
}