Reference

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.

Others

Java Peristence with JPA - A great book. If you're going to be using JPA it should be on the shelf.

JavaScript the good parts - Good reference book, the Rhino book has been recently been updated too, I have not looked at it recently but only a small portion of that book is dedicated the the JS langauge and the rest of its huge bulk is dedicated to API, for which I'd rather use the web.

Spring in Action - A very enjoyable and useful book as an introduction to Spring features.