This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.