Here are three references I've found useful:
. It's oriented to Ubuntu, but it's full of good info.
. Machtelt Garrels Intro to Linux.
. Slackware oriented, short and excellent on the basics, such as file structure, permissions, and the like.
The About dot com site has the advantage of lots of pictures. The Intro to Linux is more command-line oriented. I've kept a copy of that on my bookshelf since I started with Linux--sometimes nothing beats the printed page.