Tips on migrating from Struts to Spring MVC would be a wonderful topic for an article or blog entry, but it's not included in the book.
Why? Well, where would I draw the line? Migration from Struts to Spring...migration from WebWork to Spring...migration from Tapestry to Spring...migration from Wicket to Spring...migration from [pick a web framework] to Spring. You could probably write a book's worth of material just on migrating from Framework X to Spring, where each chapter would cover a different value for X.
It's worth pointing out here that there's no reason that you *must* abandon Struts, WebWork, Wicket, or whatever in order to use Spring. You can use Struts (or whatever you like) in the web layer of your application and use Spring for other parts. In fact, that might be the first step in such a migration: leave the web layer alone and start using Spring for things like declarative transactions or security...then if you decide to move to Spring MVC at the web layer you can do so...or not. Your choice.