"Get the O'Reilly book on the subject".
These are the books that have cute little animals or safes, or some other thing that may or may not be relevant.
I'll stick by that, with a few exception.
Their How to Setup a Webserver or the Apache book, or whatever they call it. Clearly the best book on this subject is How to Setup and Maintain a Web Site by Lincoln Stein (Addison Wesley). Although I've never met Licoln personally, I've got to tell you I don't like him. The man is a medical doctor, works on the Human Genome Project, and wrote the CGI module for Perl. Anyone who is that good at that many things annoys me, because I'm not that good at any of those. Probably working on perpetual motion in his spare time.
NFS and NiS Admin, by Hal Stern.
Don't get me wrong, it's a great book, but it's dated. If they ever update it to contain v3 or NIS+ (ok, no one really cares about NIS+) then I'll happilly remove it from this list.
For us paranoid freaks, it's a good high level AND low level discussion of cryptography.
For the love of Pete, they have a Sex for Dummies book. Guess what, if you can't figure it out, I don't want you procreating!!!
And the stuff you want to learn in 21 days people have spent years learning. And trust me, they're probably smarter than you.