I just stumbled upon this book by sheer accident (in a post from "Oldos2er") and I think it is really so important that every Linux user should at least know of it's existence!
"Linux Command Line" - 522 pages with instructions, from beginner to expert about how to use the command line.
The link of linuxcommand.org:
Click on "the book" and you will be taken to a site from which to download it.
Now the link posted by "Oldos2er":
Oldos2er, thanks very much indeed for the link!
Both links lead to exactly the same book!