Java and Ajax is really not going to be any different than PHP and Ajax...
sure JSP and PHP files are different, as well as backend .java and .php code, but Ajax is independant of the two.
What IS different are the different libraries that can assist you. Have a look at AjaxTags. Its a library for use with Java applications and makes it pretty simple to do some basic Ajax stuff...(i.e. pre-populating drop down lists, etc).
Other than than, its just javascript...javascript doesn't plug into Java any different that it does PHP, ASP, .Net, etc...
Suggestions: google, google, google, and more google. Read up, download and play with prototype.js and scriptaculous.js which are both frameworks that make working with ajax technologies a bit easier.
The head first book is great, and really doesn't cover much PHP...they only use PHP to describe whats happening on the server side.