Be advised that this is not a book you sit down and read -it is a great reference guide for when you want to know a command or config file example I guess.
Basic commands seldom change much, so AFAIK the 4th edition contains all of the 3rd edition and there's a little more about KDE and a few other things.
doddsy - there is a very good (and free) online course over at
www.linux.org and it covers all of the important basics of Linux. I highly recommend it.