Check personal page access on node add form

I need book manager to enforce personal page access so I changed hook_form_alter() to the following.

Hope somebody else finds it useful.

  if (isset($form['type']) && isset($form['#node']) && $form['type']['#value'] .'_node_form' == $form_id) {
    $node = $form['#node'];    
    
    if (user_access('add content to personal books') && !(user_access('add content to books') || user_access('administer book outlines'))) {
      if (_book_manager_get_book_owner($node->book['bid']) != $user->uid) {
        // Destroy the form
        $form = NULL;
        $form_state['process_input'] = FALSE;
        
        // Return a simple hidden field so that function doesn't fail
        $form['book_manager_access'] = array(
          '#type' => 'item',
          '#value' => t('Only the owner or site administrator can and child pages.'),
        );
      }
    }
    
    if (!isset($form['book_manager_access'])) {
      // if the book module did not already add our widgets
      if (!isset($form['book']) && user_access('add content to personal books') && ((!empty($node->book['mlid']) && !empty($node->nid)) || book_type_is_allowed($node->type))) {
        _book_add_form_elements($form, $node);
      }

      if (isset($form['book_manager_access'])) {
        // if a user has 'administer book outlines' core book may set the book widgets on non-book nodes 
        // if it does we don't want to mess with what core book did
        if (book_type_is_allowed($node->type)) {
          // remove any options we need to
          _book_manager_outline_alter($form, $node);
        }
      }
    }
  }