Bradley and DuBois have gone to great lengths to frame rap itself in a literary and historical context within American culture. Through hundreds of examples, they have managed to effectively create a chronological history of the evolution of rap as a lyrical medium since the very first rappers started rhyming over disco beats.
It's telling that all of the negative reviews of this textbook come down to nitpicking over transcription of lyrics. I can only take this as proof that hip hop culture has aged enough to where people can become so emotionally involved over the slightest variation in words. I would argue that this makes the case that much stronger for a need to give rap the thorough historical documentation and academic study it deserves. In that way, I doubt you'll find anywhere a more thorough and insightful textbook on rap lyrics that this.