This book is the step-by-step bible for hardening a Linux system, written by one of the original Unix users (and white hats).
The 800 page book is written as a cookbook to Linux (and Unix) security, with lots of explanations, details and anecdotes. It's obvious Bob knows what he's talking about and it's just as obvious that he's helped both new and long time system administrators.
The CD contains a number of easy to set up tools to detect and help correct security holes in any *nix environment. A must for any serious SA library.