Recommended Reading Material

Building Internet Firewalls (2nd ed.) by Zwicky, Cooper, & Chapman. Published by O'Reilly.
This book is absolutely essential for anyone in Information Security, considering InfoSec as a career, or even for admins of any networked application (especially e-mail). It covers expected things, like packet filtering, network design strategy, and protocol details, but also very relevant things to admins of network apps like how to build a bastion host (very useful if you're building your own secure e-mail gateway). I cannot recommend this book enough, what are you waiting for, go buy it NOW!

TCP/IP Illustrated, Vol 1 by Stevens (RIP). Published by Addison-Wesley.
Ask anyone what book you should read to learn TCP/IP and they will tell you "the Stevens book". Simply put, this is the Bible of TCP/IP. Stevens not only describes the protocol standards and how they're supposed to work, but he shows you how they actually do work in different vendor implementations (often not how they're supposed to!) by using tcpdump output. This is a highly practical way of showing the workings of TCP/IP, and it prepares you to be able to do your own investigating and trouble-shooting later on. There is no better book for learning TCP/IP than this one. If you plan to develop applications for TCP/IP networks, you might as well go ahead and get Volume 2 as well.

Security Warrior by Peikari & Chuvakin. Published by O'Reilly.
This book gives a well rounded look at security, from the machine code up. It's covers a lot of the same areas as books like Hacking Exposed, but in much greater detail. For instance when discussing buffer overflows, the authors actually walk you through attempting and perfecting a buffer overflow attack. Besides the step by step examples, this book also contains references to dozens of papers and websites that hold countless hours of further reading. I highly recommend this book to any one in IT security, who already has a good understanding and is looking to deepen their knowledge. Warning: this is not a book for beginners.

I have also written a review of this book thanks to SFOBUG and O'Reilly

Learning the bash Shell (2nd ed.) by Newham & Rosenblatt. Published by O'Reilly.
This is one of the most important books that I ever read. Knowing how to make effective use of your shell environment is critical to unlocking the power and flexibility of UNIX-like systems. You'll find that all the most effective power-users of *n*x OSs are excellent shell scripters who know how to orchestrate utility commands together to perform beautiful symphonies of systems administration. OK, so that is overly dramatic. Any way, this book gives you the foundational knowledge necessary to effectively use your *n*x system. It's written for bash, but many of the commands apply to the Korn shell (ksh), Bourne shell (sh), and also zsh (when used in bash emulation mode). I still use the book as a reference resources today.