Of course the best way to learn is from peers and internal documentation. Unfortunately, in a small IT shop the new guy may not have anyone within the organization to turn to for questions about the technical or even operational aspects of their job. However, if you have a desk with an internet connection, and your supervisors arn't looking over your shoulder 9 hours a day, then they are giving you an opportunity to learn at least the technical stuff. As for the operational part, if there is no formal documention and noone seems to know for sure, then it looks like you're going to have to write it yourself.

If you have a NetFlix account, then watch a Clint Eastwood western movie for inspiration. You tell the townpeople how it's going to work from this point forward. Just for kicks, get yourself a cowboy hat and wear it to work.