This book is probably the most important book published on the subject. It is a general look at how internet firewalls work from a conceptual point of view and their role in network security. This is not and is not intended to be a guide on how to use specific firewall products. It is an excellent overview to network perimeter security.
The book contains two basic elements: Conceptual understanding of firewalls and how to look at perimeter security generally on one hand, and detailed TCP protocol reference material on the other. I found (for the second edition) both sides were reasonably up to date, and that the industry hasn't moved far enough since 2000 to invalidate this material.
As I said, this is has very little product-specific information in it and it is not a substitute for product documentation (whether Cisco, Checkpoint, or Linux/Netfilter). However it is the best reference on the subject I have found, and it is the best introduction to network perimeter security I have seen. This topic also is universally applicable to IT fields and so should be considered to be a classic study of an important topic. For this reason, this book belongs on the bookshelf of every IT professional.