The ones on my bookshelf are:

Oracle9i UNIX Administration Handbook
UNIX Shell Programming
UNIX Power Tools
Solaris 8 The Complete Reference
Instant UNIX
UNIX in a Nutshell
UNIX Made Easy

Holy cow, with that many UNIX books, I just realized I'm on the verge of being a UNIX geek. Gotta remember that the upshot is "Chicks dig UNIX."