Unix Unleashed
by Robin Anderson (nee Robin Burk), Andy Johnston
Covers the knowledge a system administrator needs from the basics through to advanced stuff - the writing style is clear and concise and doesn't treat you like an idiot. The CDRom includes CVS, Gimp, Grep v2, VIM and a ton of other useful stuff.

Now in it's 4th edition the book has been tweaked and improved over the years - if you're only going to buy one UNIX book pick this one - it's quality stuff.

UNIX in a Nutshell
by Arnold Robbins, Daniel Gilly
Like a dictionary, Unix in a Nutshell helps you find what you need, even if you're not exactly sure what you're looking for

This book is the standard desktop reference - if you don't already have a copy - buy one now!

UNIX Power Tools by Jerry D Peak et al
Organised quite unlike a command reference - each chapter in this book describes the different ways of performing a task. So the section on Wildcards has 10 ways to select a group of files - only two of which involve the grep command.

UNIX System Administration Handbook
by Evi Nemeth,Garth Snyder et al
An expensive but comprehensive unix reference. It explains the 'Why' rather than the 'How' which may not be to everyones taste. It's a good book but I would read the other titles listed above before tackling this one.