I'd recommend reading some books published by O'Reilly.
Learning the Unix Operating System is good if you are a beginner. If you're comfortable navigating around Unix at the command line (creating directories, copying files, etc.) but want to really leverage its power, read
Learning the Korn Shell. You'll probably also want to learn as much as you can about using
vi,
sed, and
awk.
Reading about it only takes you so far, however. The best way to learn, in my opinion, is to immerse yourself. Get an account on a Unix box at work, install Unix/Linux on a cheapie personal computer, find a free account online, etc. Force yourself to use Unix for something that you normally would use Windows to do, and don't give up until you get it to work. You'll learn a lot that way. And try to avoid using the graphical user interfaces (if available) in Unix as much as you can. Learning to work from the command line is good.