I agree with Web31337, man pages and google are your best friend. If you don't know (I learned today
) Google.com/linux will give linux specific searches and may help with your searches. Books are good to keep on hand, but trial and error are your best sources for knowledge. Set up a box at your house and start playing with it. Set up everything, DNS, proxy, dhcp, web server, ssh, ftp, user accounts etc. I found that just a good all around book was best. I bought this
a couple of years ago and it is a good reference although there may be an updated version now.