Most of the book is concerned with defining terms, and telling you to think straight, to avoid VanityFear. If you have been following his CryptoGram? newsletter, there is little new in the book. If you feel that airport security is costing a lot and doesn't make flying more secure, but can't really explain it because you are lacking the vocabulary, then this book is for you. It will allow you to reason about security not because it explains how security works, but because it defines the terms used to reason about security, and it gives you a few (simple) examples.
Terms introduced, defined, and explained include system, security, safety, attackers, attack, defenders, defend, assets, threat, risk, countermeasures, profiling, detection, prevention, reaction, mitigation, recovery, forensics, counterattack, rules, procedures, player, agenda, and most important of all: trade-off. He wants us to think about security countermeasures, consider all the ramifications, and then decide: Is the trade-off worth it?
Chapter 15 is the most interesting part of the book: Fighting Terrorism takes a look at security measures implemented to prevent terrorism.