
A complete guide to setting up and running a TCP/IP network for administrators. Covers Linux, BSD and System V TCP/IP implementations.
UNIX Systems Administration
The Perl Cookbook
DNS and BIND
AWK
and too late for me to learn the basic, but valuable nonetheless: The Practice of System and Network Administration.
Flipping through the Smalltalk books was an eye-opener back in the day.
There's an amazing but little-known book in the same printing style as K&R about systems software from the MCC consortium, with tons of C source code.