Learning Command Line

One of the great features of the Linux command line, is not only the features, but the built in documentation. You can access the manual or "man page" of any command by adding "man" before the command you want to learn about. Example:

man top
man grep

The man page will tell you everything you need to know about a specific command. If you are looking to learn more, I would suggest any of the Oriley books, they have everything from pocket references (that list the commands and uses), to full featured books that will walk you through everything there is to know.

Myself, I just use google..especially for ubuntu specific questions.