For an introduction to Ajax i would recommend the "Head Rush Ajax" book. Part of the HeadFirst series.

Publisher: O’Reilly

Ajax book covers the Ruby on Rails with Ajax, but does not cover some other Ajax topics such:

1. AjaxTags- are used in a JSP page to send an Ajax request.

http://ajaxtags.sourceforge.net/

2. Ajax4JSF-integrates Ajax with JSF.

https://ajax4jsf.dev.java.net/nonav/ajax/ajax-jsf/