Struts: The Complete Reference (2nd Edition) has been recommended to me. I've bought it but finding it a bit of a struggle.
It is from 2007 and the Tag Library Descriptors seem to have a completely different uri now which causes issues in the sample application.
I have managed to get it working but it took me 4 hours longer than it should.
Although it won't be forthcoming because Struts2 is out now, this book really needs updating.