One of the big problems with Struts 2 is that it has nowhere near the market penetration that the original Struts has. As a consequence, there is not a whole lot of resources to draw upon when learning.
First, bookmark the
Struts 2 home page. There's links off that page to download the library and to tutorials.
There is only one print book available (AFAIK): Ian Roughley's
Practical Apache Struts2.
There's a couple of smaller PDF books: Ted Husted's
Struts 2 From Square One and Ian Roughley's
Starting Struts 2.