Probably the most important book in any UNIX or LINUX administrator's library, this 1200 page book covers the essentials for every aspect of administration. Everything from basic admin through security, T/CP and networking through building new kernels, is covered in very easy-to-read detail.
The most exciting thing about with this book is the set of comparison charts in each topic showing the AIX, Solaris, IRIX, Linux/RedHat, Linux/SuSE variations on a command.
Published in 2002, it is slightly behind the current versions of the operating systems. That should not be an issue as the writing style carefully guides the reader into understanding the basics, allowing anyone to quickly derive variations for the latest changes.