I forgot to mention that that book also has examples of
AJAX form code that can be downloaded from the book's page.
The problem with traditional Form data with complete page reload is that you have to retype everything if you submit wrong data to the form.
Because of partial page reload you need not do that using an AJAX form. That incereases user experience on your site, and indirectly increases traffic.
In addtion it describes how to write AJAX applications for disabled people that is of value in itself.
The idea is to write own CSS rules for disabled people that are hidden for able surfers.
Highly reccomended.
If you are new to the concept of AJAX, here is an
PHPBuilder.com, the best resource for PHP tutorials, templates, PHP manuals, content management systems, scripts, classes and more.
introduction.