Operating System Internals for me. here is the Book http://www.amazon.com/Design-Implementation-FreeBSD-Operating-System/dp/0201702452/ref=sr_1_2?ie=UTF8&qid=1289469941&sr=8-2
While being a self taught sys admin is great, learning the internals of how things work can really extend your knowledge beyond what you may have considered possible. This starts to get more into the CS portion of things, but who cares. It's still great stuff to know, and if you know this you will really be set apart. Im not sure if it will help you directly as a sys admin, but may quench your thirst. Im both a programmer and unix admin, so I tend to like both. I own or have owned most of these and enjoy them greatly. You may also consider renting them or just downloading them. I can say that knowing how thing operate internally is great, it fills in a lot of holes.
OS Internals
While you obviously are successful at the running and maintaining of unix like systems. How much do you know about their internal functions? While reading source code is the best method, some great books will save you many hours of time and will be a bit more enjoyable. These books are Amazing The Design and Implementation of the FreeBSD Operating System
Linux Kernel Development Advanced Programming in the UNIX Environment
Networking
Learning the actual function of networking at the code level is really interesting. Theres a whole other world below implementation. You likely know a lot of this.
Computer Networks
TCP/IP Illustrated, Vol. 1: The Protocols
Unix Network Programming, Volume 1: The Sockets Networking API
Compilers/Low Level computer Function
Knowing how a computer actually works, from electricity, to EE principles , through assembly to compilers may also interest you.
Code: The Hidden Language of Computer Hardware and Software