Last summer our reading group here on Bainbridge went through Bruce Schneier's Secrets and Lies: Digital Security in a Networked World. I found the introduction, to be one of the most useful parts. Here Schneier discusses the economic reasons that led him to convert Counterpane into a security service company. Aside from that there's a lot of high level stuff that is very education for lay people (but too light for people with some background). That material also serves to show how bad the computer security picture is. The other parts that I found interesting were chapters 19 and 21 which deal with threat modeling and attack trees.

If you don't have a security background, this book is a good introduction. If you do have a security background, you should keep this book around to give to your non-technical friends and co-workers. Schneier now has a blog where he's posting the contents of his excellent CRYPTO-GRAM newsletter, as well as other content.