There is documentation for views 3 (sort of) in the advanced help.  But if you don't want to implement that module there's a copy on line, the page you are looking for is at http://views-help.doc.logrus.com/help/views/api-tables which helped but is a bit confusing (and possibly  not complete), but when supplemented by this issue, http://drupal.org/node/370013, especially the PATCH in it, I was able to get something working using hook_views_data.

 

Gregg