I read this a couple months ago and failed to take it with me to Seattle, so I've lost the notes I took on it, but it at least bears mentioning.
He proposes looking at a security problem/solution using the following steps:
1. What assets are you trying to protect?
2. What are the risks to these assets?
3. How does the proposed security solution mitigate those risks?
4. What other risks does the solution cause?
5. What trade-offs and costs does the solution impose?
It's a good introduction to some of the principles and key terms in security (at least, from what I can tell, as someone who knows very little about the field). He uses examples of national security throughout the book, essentially telling readers that terrorism isn't as much of a threat as everyday dangers like heart disease and car accidents, and that the current solutions do not mitigate the risks well. What I liked most about it was that he can frame anything in terms of a security problem and explore it in-depth (including a lot of things I wouldn't normally have thought of in that way, such as maintaining a population of honeybees), which puts it in the category of "books that help you learn to think differently". If I were put in the position to teach an undergrad-level course on computer security I would make it required reading in the first couple weeks, just to get students in the right frame of mind to think about security problems and solutions.