function get_book_comment($book_id) {
        global $wpdb;
        $table_name = $wpdb->prefix . 'posts';
        $query = "SELECT `post_content` FROM $table_name WHERE `ID` = $book_id;";
        $result = $wpdb->get_results($query, ARRAY_N);
        if (!empty($result)) {
            return $result[0][0];
        }
        else {
            return '';
        }
    }