Reddit mentions of Learning the UNIX Operating System, Fifth Edition

Download Virtual Box.
Download a few Linux Distros - Ubuntu, CentOS, etc. Ubuntu is rather user friendly. Get the server version, not desktop. (Or get both, but you want experience with the command line, and the server version will force you to work with the command line.)

O'reilley books are great. Very dry, but reliable sources of info. Go "used" for now, as they can get expensive when you buy too many.

Learning the Unix OS is a good intro.

Also, this book is a large overview of linux sysadmin: Essential System Administration by Frisch

A Bash book would help, as well.

Oh, and the "man" command. On the command line, run "man anycommand" to bring up manual pages on any command. You can run "man man" or "man bash" to get started.

Best of luck!

Edit: typo

This is a good book. You should supplement it with something like this because one of the big things about UNIX (and its derivitives) is the underlying philosophies around processes, files, and CLI tools.

I would heartily suggest putting linux on a PC (any old PC from the last decade will do, honestly) and learn by playing around on it. Ubuntu, Fedora, any free version will do. You could also do this in a VM on your windows or mac.

For those pushing OSX -- yeah, it's got UNIX/BSD under the covers but there are also a lot of differences you have to dig under to get to a "plain" UNIX environment with X11 etc. (I've been playing with UNIX on mac since the Mac II, understand this isn't linux prejudice talking). It'll be a lot easier to install a linux variant fresh somewhere.

http://www.amazon.com/Learning-UNIX-Operating-System-Fifth/dp/0596002610/ref=pd_sim_b_2

So basically read the O'Reilly books: Learning the Unix... and Unix Power Tools.