Have you searched the web for Struts 2 books? I can think of three off the top of my head: mine (not just Struts 2), Struts 2 in Action (quite good), and... that third one whose title I don't remember.
Ah, yes, that's the one. It was actually pretty good.
One issue with S2iA and S2D&P is that they only cover Struts 2.0, but a good portion of the information is still valid. I have both, along with Roughley's free InfoQ book and the Web 2.0 book (which I didn't care for as much).