No..didn't miss your question. I just am not able to read these all day long...I only get a few hours in the evening and a few random hours through the day to look at them...just now getting to yours.
But...yes, the book starts out assuming you don't know Spring at all. But you can turn to any page and pick up at the point where your experience allows. I wouldn't say that there's a specific chapter to start on if you're already experienced with Spring 2.5...but
you should be able to scan and find where you should start.
Yes, the web front end (horrible title...maybe I'll fix it) is Spring MVC. The REST chapter will also cover some Spring MVC.
And yes, the "Securing Spring" chapter covers Spring Security in quite some detail.