I'm not one of the authors of that book, but I'm fairly certain that
they haven't updated it since the original publication. I hadn't heard
that Springer had released a paperback edition either, but if they did
I don't think it has any new material.
As a non-author, I can strongly recommend their book. Some of the
material is a bit dated now, but it's still got a ton of good content
and summarizes a lot of important work.
I should also plug the 10th anniversary issue of the journal of
Genetic Programming and Evolvable Machines which is available for free
in its entirety until the end of July (just a few more days!)
http://gpemjournal.blogspot.com/2010/07/gpem-1134-anniversary-special-issue.html
This includes, among many other things, a lengthy summary of the state
of GP theory.
Thanks!
Nic