In Accessibility and Usability issues with AJAX, Joe Dolson provides a think piece detailing the issues and solutions for accessibility and AJAX. This analysis is useful information for anyone using or teaching AJAX.
This is not a technical article. You will not learn how to code AJAX by reading this; either in an accessible and usable fashion or otherwise. This is a conceptual article. It will run through basic user-interface issues with AJAX (and other rich media). These are the reasons that AJAX functionality can be a problem for users — if you consider these issues carefully during development, it should greatly enhance the usability of your end product.