Ahhh man, I have an excellent academic book on the subject at home but I'm currently at work and can't remember it's title for the life of me. I will get to it this evening for you.

I recommend Bomb the Suburbs by William Upski Wimsatt. It's not about rap per se, but more about the beginnings of the culture of hip hop. Goes into detail about how break dancing, graffiti, and rapping all coincided to create what we call "hip hop" today. Relatively short read, easy to understand, not pretentious, but very informative. I highly recommend it.