Counter Hack Reloaded: A Step by Step Guide to Computer Attacks and Effective Defenses (Radia Perlman Series in Computer Networking and Security) by Skoudis, Edward, Liston, Tom [23 December 2005]
Excellent overview of computer and network security concepts, issues and practical user actions. The book summarizes key under-the-hood concepts as clearly and simply as possible. Anyone who wants a fundamental understanding of the risks of connecting to any network should read this book.
Very informative with a decent background to bring you up to speed on how to hack. I learned a great deal from this book and found his suggestions very useful in security operations.