Books

Admin.com
The companion site to Unix System Administration Handbook and Linux Administration Handbook.
http://www.admin.com/

Go Solo 2 - The Book
Authorized guide to version 2 of the single UNIX specification.
http://www.unix.org/gosolo2/

Linux Administration Made Easy
A published book made available online. Covers adding users to "Enterprise Computing."
http://tldp.org/LDP/lame/LAME/linux-admin-made-easy/

Linux Quick Fix Notebook
Simple to use systems administration guide for setting up a Linux based website. Lots of practical examples and troubleshooting tips. This book was written specifically for busy systems administrators who need to quickly finish a project with just the essential background information for it all to make sense.
http://www.quickfixnotebook.com/

O'Reilly Linux/Unix Center
Features Linux and Unix books, resources, news and articles.
http://linux.oreilly.com/

UNIX Unleashed, System Administrator's Edition
Online book. Unix basics, shells, administration tasks.
http://athena.vvsu.ru/docs/unix/unix_adm/toc.htm