public function get_details($id = NULL, $format = null) {
        $book = Model_Book::query()->where('id', $id)->get();
        $data = array();
        $data['title'] = $book->title;
        $authors = $book->authors;
        foreach ($authors as $a) {
            \Arr::insert($data['authors'], $a->name, 0);
        }
        $genres = $book->genres;
        foreach ($genres as $g) {
            \Arr::insert($data['genres'], $g->name, 0);
        }
        $data['series']['name'] = $book->series;
        $data['series']['number'] = $book->series_id;
        $data['time_updated'] = $book->created_at;
        switch ($format) {
            case 'array':
                return $data;
                break;
            case 'xml':
                return Format::forge($data)->to_xml();
                break;
            case 'json':
                return Format::forge($data)->to_json();
                break;
            case NULL:
                throw new MissingParamException('This function expects a format to be given as parameter 2');
                break;
        }
    }