Note on books - Spring in Action 3rd edition hasn't been released yet - there is an early access version available through the publisher, but it is not complete.
a.) The biggest changes for Spring 3 are annotation based configuration. Spring 2.5 code will still run in Spring 3. I'd download Spring 3 and use it, but as for learning materials - don't worry too much about Spring 2.5 vs. Spring 3 - the biggest difference will probably be doing all configuration through XML instead of annotations - and you still need to know how to do XML based configuration anyway. There are bigger changes in Spring MVC - the web framework - in Spring 3 it's now all annotation based, Spring 2.5 used a hierarchy of controller classes.
b.) Not sure - I'd guess most products still being developed would upgrade to Spring 3 though... like I said in a. - Spring 3 will still run Spring 2.5 code. I can't think of a good reason for new development to prefer an older version of Spring (the only reason I can think of is if if you were targeting a pre-1.5 JDK - no annotation support).
c.) The
Spring MVC Step-By-Step tutorial is the best example I can think of - unfortunately I can't find an updated Spring 3 version of this tutorial.
