public function setAuthor($author, $sort_key = '')
    {
        $this->creator = $this->metadata->appendChild($this->createElement('dc:creator'));
        $this->creator->setAttribute('opf:role', "aut");
        $txt = $this->createTextNode($author);
        $this->creator->appendChild($txt);
        if ($sort_key) {
            $this->creator->setAttribute('opf:file-as', $sort_key);
        }
    }