What most surprised me is that the book is written based on Struts 2.1, which is in beta phase. Does that mean that there can be minimal changes that can make the book's code not to be "fully" functional?
It's possible there would be very minor incompatibilities, but it's not terribly likely. We've been largely feature-complete in Struts 2.1 for some time; the delayed release has been due more to time commitments of myself and the other committers.
What are the example result types showed in the book?
The book covers the standard result types and develops a Markdown custom result type as a simple example.
What's the coverage of Struts 2 with AJAX? Does it include JSON?
The RIA chapter briefly covers simple use-cases of the "easier" Dojo-based Struts 2 Ajax tags (I was hesitant to include more for a variety of reasons). It also covers the REST plugin, including the consumption of both JSON and XML response data through various means.
Dave