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 (2005) Paperback
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.