The Linux® Command Line, by William E. Shotts, Jr., has been released under a Creative Commons license, and is available for free download -- unfortunately, in PDF format only. I'd much prefer an epub, but this still works.
The download link is
<HERE>.
As someone who learned on old-school UNIX® -- back before graphic windowing environments were common -- I've had some fun using commands in the terminal window.
This guide is applicable to many (I'm tempted to say "all," but I guess there are always exceptions) Linux "distros" -- I use Ubuntu 10.04 LTS on my netbook.