Networking with Linux?

I have a friend who want's to hire me for basically editing and keeping his network/servers up to date and uniform. The only problem is that I don't know a lot about working with servers, especially on Linux. Anybody have some good sources on this topic? Also and info on Linux commands would be great.

What distros is he running? what are they used for? There's tons of resources in forums, and many distrobutions host their own documentation. Other than that hit Amazon, and start reading. 

A lot of skills with linux comes with experience. Most courses and training sessions and certifications are just rubbish and mean nothing in linux. There is no one single "linux", it's all pretty custom and different, that's why it's so powerful. Do you have any UNIX exposure whatsoever? How long have you been using linux as your desktop operating system? What kind of servers would it be, and what would they run? Is it a hybrid network, with windows consoles on them, or won't you have to fight malware. Are these high security servers, or just gaming servers or something? ...

Big emphasis on what they are running and what they are used for.

It's Ubuntu based, and it's basically storage for his business, with firewall routers. And the only experience I have with Linux is with Ubuntu and Mint, although I heavily relay on the gooey interface.

info coreutils