iptables

Designing and Implementing Linux Firewalls with QoS using netfilter, iproute2, NAT and L7-filter
$39.99
It is very well written

You will learn about NAT and filtering. Maybe you will need read more about QoS, but like introduction it is fine.

Excellent book. It shows you about small-medium-large networks configurations.

Regards,

Linux iptables Pocket Reference
$9.95
Last year, I was forced to become a fly-by-night system administrator. I worked for a small, local startup as its web developer, but was thrust into a sysadmin role when my boss decided to host a website on a server in our office. I was developing the site on our Ubuntu server, but was learning how to secure the server on the fly. This reference, out of all the other books I read and sites I visited, had the most bang for the buck.

It's short and sweet. It describes what you should know, and gives you a reference for dealing with iptables syntax, and that's it. No flowery text, no colored pictures. Just simple "This is how to do X."

If you're a sysadmin, especially if you're just getting your feet wet, get this book. It's cheap, it tells you what you need to know, and it fits in your pocket. What's not to like?

Linux Firewalls: Attack Detection and Response with iptables, psad, and fwsnort
$49.95
This was a great book! Helped me understand networking, linux, and iptables better. Michael Rash has written some great software, and this is also a definitive guide on it.

LINUX Network Administrators Guide: 508 pages
$25.80
The LINUX Network Administrators Guide spells out all the information needed for joining a network, whether it's a simple UUCP connection or a full LAN with a Linux system serving as a firewall, an NFS or NIS file server, and a mail and news hub. This book touches on all the essential networking software included with the operating system, plus some hardware considerations. Topics include: Introduction to TCP/IP, Configuring network and serial hardware, Domain Name Service, Serial line communications using SLIP and PPP, NIS and NFS, Taylor UUCP, Administering electronic mail, including sendmail and Exim Administering Netnews, including INN and several news readers, Firewalling using ipfwadm, ipchains, and iptables (netfilter) Masquerading and accounting, IPX configuration for a Novell Netware network.