Here's one I'd recommend. It's probably the newest Struts book on the market, and it also includes content on Ajax, which is not only my hometown (shout out to the 683!), but is also the hottest thing on the server side.
Pro Apache Struts with Ajax Also, the Struts reference is good. Almost too much, but too much is better than not enough. It is actually a great reference:
Struts: The Complete Reference by James Holmes I also have a few
Free Struts Tutorials on my website, which is probably a good place to start.
Happy Struts!
-Cameron McKenzie