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 '';
		}
	}