Defensive Design for the Web
People who have web sites that have lots of forms of their pages, Defensive
Design for the Web by 37signals is a must read. It goes through error messages,
checkers, and how forms should behave. There are other general issues covered
but the book really is about good practices with forms. It is published by
New Riders and the bookstore should have a copy or two if you want to check
it out.