I had an eagerly awaited delivery arrive today:  the brand spanking new Windows Server 2003 Resource Kit.  I've been using NT resource kits since the NT 3.5 days, and have always thought of the books as my own personal Gazoo.
The Great Gazoo, for those unfamiliar, was a character on the Flintstones cartoon.  He only appeared in a handful of episodes, but he was basically this alien from the future that only Fred and Barney could see, and offered them advice.
The Resource Kit fills much the same function.  It sits unobtrusively on a shelf until you're in a jam, and then poof, it's there when you need it. 
There's a huge wealth of info in here.  Security gets its own book, as does group policy.  Russinovich's excellent Windows Internals is now included.  The troubleshooting guide is well indexed and includes lots of really handy information, like common STOP error messages, allowing you to quickly reference and identify the cause of that ugly blue screen of death your domain controller is exhibiting.  Turns out that IRQL_NOT_LESS_OR_EQUAL is likely a hardware failure, for example.
The Performance Guide is full of really good info too.  If you're only using Perfmon (or worse, taskman) to judge your server load, you're making a big mistake.
The Automation Toolkit is the best kept secret in the set.  Did you know that nearly everything you do through the UI in Windows Server 2003 can be done from a command line?  The toolkit book is full of sample scripts and utilities you can use to automate all those tasks admins perform on a regular basis.
The set also includes a tools CD, filled with utilities for doing all kinds of esoteric things that turn out to be really handy to do. 
Yeah, if you don't have one of these books in your Admin arsenal, you're playing from behind.  Get it, read it, grok it.  really.
Tell them Gazoo sent you.