public function getQueryUrl()
    {
        if (isset($this->_url)) {
            $url = $this->_url;
        } else {
            $url = Zend_Gdata_Books::VOLUME_FEED_URI;
        }
        if ($this->getCategory() !== null) {
            $url .= '/-/' . $this->getCategory();
        }
        $url = $url . $this->getQueryString();
        return $url;
    }