My project is based in java. As the server side code is in PHP and I don't know PHP, can this book help me understand the complete server-client cycle?
You can use AJAX with jsp pages or servlets - just use the response.getWriter().write(whatYouWantWritenOut) method. The JavaScript/client side code is the same.
The focus in this AJAX PHP book is AJAX as a technology and the server side techniques in PHP is kept to a minimum. I am sure that by reading this book you can easily convert the server side examples to Java (Servlets/JSP).