Reference

Web Sites

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

Books

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.