For Linux system Administration?
Beating your head against a small Linux LAN, by Trial and Error
But seriously, The Linux System Administrator guide is excellent, all four times I've had to pick it up, but far too broad on most subjects. For instance, if you want to know nameservers: DNS and Bind. For webservers: Apache, the definative guide. Just for the basic system: Running Linux by Matt Welsh. These are all O'Reilly books, so I may seem a little biased. Basically its hard to beat O'Reilly texts, although they're always on the pricy side, but I work in a library, so I just recall them out from under under-grads.