Guest book is one of the essential part for any website, To add a guestbook on your drupal site you can use the guestbook module, the problem you would face is adding a captcha to this now captcha is also very essential to control spam now how to add captcha its very simple just execute the following query

INSERT INTO `captcha_points` ( `form_id` , `module` , `type` ) 
VALUES 
( 'guestbook_form_entry_form', NULL , NULL );

and you are done now go to /admin/user/captcha on your site and enable captcha on guestbook this would look something like this