This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Security is a big field. As with just about everything, the more you know, the better. Since you're posting in the servlet forum I'm assuming you're interested in web security. The Tomcat FAQ has some information on how to set that up, and the Security FAQ has some more general stuff.