nmap

Penetration Tester's Open Source Toolkit, Vol. 2
$59.95
Chris does a great job of laying out the steps and procedures in an incremental format. Also, the layout of the book (Chapters) is in the order one would conduct a penetration test, which is very useful to someone that is learning the trade. I liked the tie-in with the backtrack cd. The command line screenshots make it a very usable reference guide. I imagine this book will stay within arms reach for a long time.

Netcat Power Tools
$59.95
This is a must have book for anyone seriously interested in using netcat as a network/system tool.

Nessus Network Auditing, Second Edition
$59.95
This book covers the new features included in version 3 of Nessus, including the file content and security check audits that are only available through a direct feed subscription. Other books make cover Nessus but do not cover these new features. I use Nessus with privacy and security audits. This book is a must if you are using Nessus in a professional capacity. I keep track of local security law changes for the United States, and other background regarding using Nessus as a consultant at computersecuritylaw.us. I also recommend this book to non-Nessus using computer security officers, with increased cyber-attacks one should really consider this or other similar product as part of your computer security program.

Hacking: The Art of Exploitation, 2nd Edition
$49.95
The biggest problem I had with this book is that the Live-Cd had some sort of defect. Regardless of what I choose in Grub, it always came back with the same error. Starch Press' response was to download and reburn the cd, but that didn't change anything.

I was able to do some of the examples in Ubuntu, which I already had, but b/c of particulars in gcc and the environment in general, I wasn't able to get many of the examples to work.

The book itself is good, although requires that you sit down and work through the problems in order to understand them.