Reference

Version 5.1 by Ken McWilliams on 2012/04/04 01:40

Web Sites

struts.apache.org - note the getting started, tag reference and plugin registry sections.

OGNL - The EL typically used in struts2.  It is so simple people overlook its more advanced features.

StackOverflow - The SO community has answered a lot of common (and not so common) questions, it is a great resource.

JSTL - It's nice to know you can use these with struts2 as well.

Books

Struts2

Struts2 in Action - I personally find this book quite useful

Apache Struts2 Web Application Development - I personally find this book quite useful too

Practical Apache Struts 2 Web 2.0 Projects (Practical Projects) - No experience with this book

Starting with Struts2 (pdf)- It's free, It's Online... I personally would do a couple tutorials and then read this book, if you're looking for something stronger I'd suggest one or both of the first two books.