Undefined property: stdClass::$book

I noticed PHP complaining on pressflow:

WD php: Notice: Undefined property: stdClass::$book in advancedbookblocks_nodeapi() (line 564 of                            [error]
/path/to/drupal/sites/all/modules/advancedbookblocks/advancedbookblocks.module).

and it seems to me that

function advancedbookblocks_nodeapi(&$node, $op) {
  
  $bid = $node->book['bid']; 
  // doing its thing
}

should make sure that we are on a book node before it does its thing, like so:

function advancedbookblocks_nodeapi(&$node, $op) {
    
  //check to see if node is a book
  if (!empty($node->book['bid'])) {
    //doing its thing
  }
}

I've attached a patch to the point.