Source Engine tips? Thread

I intend to make this mod:
Moddb.com

So I'm taking the time now to go over the Source engine's code and try to get a feel for how the whole thing's structured. The Valve Developer Wiki has been very helpful, and their forums are fantastic, but the population's a little sparse.

My question for all of you is:
What objects (headers/code) should I be looking at to implement the functionality I'd like to achieve? I'm not asking you to tell me how to do each of these things I'd like to achieve (though tutorials are welcome and appreciated), I'd just like to know where to look.

More to the point:

Which objects are key to the overall structure of the Source engine? What makes up its backbone?