A Guide to Ubuntu Linux
Top Ubuntu Tools
One tool that gets too little coverage in most Linux books for new users is OpenSSH. As the administrator of Linux servers, you'll use it for all kinds of remote administration tasks. This book, though, gives it a whole chapter, which should get you started on managing keys, logging in securely without a password, tunneling protocols that you want to secure, and other essentials. (If your first new Ubuntu box is a server, and you'll be ssh-ing in from your non-Linux system , check Rick Moen's ssh client list for the software you'll need on that end.)
Sobell covers the new Upstart init system, featured in Ubuntu 6.10 and later. If you have never set up services to run automatically at boot, upstart is a more featureful way to do it than the old Unix System V-based init scripts, but much of what you read in a book about Unix, or other Linux distributions, won't work. For example, you can't just put a line in /etc/inittab to have init restart a program if it dies. A Practical Guide to Ubuntu Linux will save you some time over learning init the old way, then discovering that you have Upstart instead.
Beyond the Basics
Besides chapters on the basics, there are good introductions to server tasks such as mail, file server setup, firewalling and NAT, and web serving. Yes, this is a good intro for learning Linux at home, but you can make your Linux box at work earn its keep without going back to the bookstore. A shell scripting chapter will help you even if you don't plan to automate any of your own tasks that way yet--you'll probably need to figure out someone else's shell script at some point.
One of the most important Linux tools for new adminstrators, nmap, gets only a brief mention. Running nmap is a useful habit to make sure that you're running the server software that you think you're running, or that a firewall is set up correctly. I use it to check firewall and network address translation setups--it would have been helpful to mention nmap in the chapter on firewalling and NAT.
And there's plenty of material on Apache, but many of today's new Linux boxes are being deployed in order to run a PHP application, and new users could really use some material on how to set up PHP correctly and securely, and make sure the basics are working before installing that nifty new Wiki or web board application.
But you can't add everyone's favorite topics to a book, or they wouldn't be able to bind it. Work through the core material here, plus one or two of the server tasks, and you'll be ready to get some meaningful projects done with Linux, and ask or answer questions on mailing lists to expand your knowledge.