The most important thing is to get comfortable with the command line.
When you don't know how to use a command line app, type: app --help
(replace "app" with the name of the app obviously) and if this doesn't give you enough information, type 'info app' to see the manual for that tool. The 'info' manual reader isn't very nice, but the documentation for GNU software is excellent, so just get used to info. GNU Emacs has a nicer info viewer, if you don't use emacs already, you might want to give it a try. I've listed the basics on this webpage:
To learn about the history of GNU/Linux, you could read "Free as in Freedom" by Sam Williams. This is a biography of Richard Stallman, the guy that created the operating system. It's available online here:
And if you're using Debian, you can view the Debian user/administrator manual here:
(most of this info will be useful with other distros too.)
Finally, to learn about why the operating system exists, you can read about the GNU philosophy here: