Make sure you start at the beginning. There's nothing worse than so-called rap scholars who don't know what they're talking about. I'm not sure if you've read it, but you should check out Can't Stop Won't Stop. It's a book that came out about 10 years ago and it goes into depth about how Hip Hop started in the early years. It goes up to about the Rodney King riots so it's really just about Old School Hip Hop. great read.
There's also a book that Vibe put out about 10 years ago, it talks about 80s and 90s rap but their chapters on old school rap are suspect and not entirely accurate.
My point is that you can't write about Hip Hop without discussing the old school.
A good website to use for references is thafoundation.com