Secrets of Computer Espionage: Tactics and Countermeasures
by Joel McNamara
This is a good overview of computer security issues. It's a bit dated at this point in the game but many of the ideas are still concerns today. The book lists a number of great security related URL's (Although some are offline now or have changed addresses), most of which are jems. The author does a good job of explaining the weaknesses in computer systems in a general way but doesn't cover the more modern OS's like WinXP/Win2000/Win2003. He uses a spy vs counter-spy model to explain his points which I found entertaining.
Recommended!