This was one of my first linux security books and i have found it to be my most refered to by a long way, however it is now pretty dated as it is over three years old (when 2.2 kernel and ipchains were new). If you are considering purchasing this book i would sugest you look at the updated second editon, however you cant realy go wrong with this book as it will give you and excelent grounding in the priciples of securing you linux workstation/server and with a little web research you should be able to apply these ideas to the lateest versions of the software.
This book is written very precise, and is a valuable reference guide to every topic covered in it. It brings you up to speed with everything you need, and want to know about linux related security matters. Why only 4 stars, when it is so valuable? Well, for one and only reason: the successor is due to come out some time this year, and it will certainly provide more up-to date information. I can only recommend this book to anyone interested in security, for private or business purposes. Excellent.