public function get_list($start = null) {
        if ($start == NULL) {
            $offset = 0;
        } else {
            $offset = $start * 30;
        }
        $books = array();
        $book = Model_Book::query()->limit(30)->offset($offset)->get();
        foreach ($book as $b) {
            \Arr::insert($books, $b, 0);
        }
        return $books;
    }