The UNIX System Administration Handbook (third edition) is great reference and tutorial for users and system administrators alike. The third edition (released in 2001) has been expanded and updated to reflect the types of modern UNIX systems in common use nowadays (the book covers each topic with specific notes on the differences and peculiarities of each of their four example systems: Red Hat Linux, Solaris, HP-UX, and FreeBSD).
The thing that makes this book particularly valuable in my opinion is that as well as providing a decent functional overview of a whole host of technical matters that are likely to confront any system administrator, the book also contains a decent amount of stored wisdom on the human aspects involved, things like managing user requests and keeping the users pacified and productive to things like how to present technical matters to pointy haired bosses and how to create an acceptable use policy that is fair and well balanced.
On the more technical side, the book covers a lot of topics, and while it doesn't go into incredible depth on all of them, it does provide plenty of leads if you want to track down more detailed or current information. The technical topics covered include:
There is also some general discussion giving a quick summary of the
UNIX family tree and some of the reasons that various things have evolved to their current state, as well as a bunch of great
cartoons and diagrams to boot.
The book is published by Prentice Hall, ISBN 0-13-020601-6. The primary authors are: Evi Nemeth, Gary Snyder, Scott Seebass, Trent R. Hein.