I'll try and sell you on GWT and my book.
First, are you sold on Ajax applications in general? They have the advantage of giving your users/customers a better user experience than they would get on a traditional web page (no waiting for page refreshes). They can perform better than traditional web application since they don't need to load as much data, can reduce the number of server round trips, and can move some of your application state to the browser relieving your server load somewhat. They're also easier to deploy than desktop applications with less dependencies.
Ok, now you want Ajax for your client side. If you know Java and want something that's easy to learn you should use GWT. GWT provides a library that lets you use the Java tools you're familiar with (JUnit, Eclipse, Tomcat, etc). You can also use the Java software development techniques you and your team are comfortable with.
Ok, so now you're thinking that GWT may be pretty good. Should you buy my book? You can find a way to get started with GWT on their website. They have a few sample applications and documentation for their library. This is all I needed to get started. However I've been working with GWT for a year an a half so far and have learned quite a bit more. You need to put the toolkit documentation in context. I put the user interface library in context by overviewing it's use in many applications in ch.2. I should the many techniques of communicating with a backend using the GWT tools in ch.3. I show you software engineering techniques for GWT in ch.4. In chapter 5 I show advanced techniques on using the toolkit effectively including server performance considerations. For just this first half you get quite a bit of GWT experience in about 300 pages. The second half goes into detailed application development case studies on 5 sample applications where touch on just about every way that GWT can use a browser and interact with the web. It's a great book and it has something for every stage you're at when you learn GWT.
Hope this helps!