Reddit reviews Book of Rhymes: The Poetics of Hip Hop

Yeah that one's really great. I'd also recommend Can't Stop Won't Stop like /u/courage_wolf said, as well as Book of Rhymes: The Poetics of Hip-Hop

Rap IS poetry man, at least the good stuff. I've always loved Doom for the rhyme schemes and original metaphors with lines like
"A dead mouse is how he catch a phrase / in paws plays with it / kill it, and eat of it for days, get it?"
He uses a lot more tools than a lot of rappers. Aesop Rock is another guy who borders on gibberish but after enough listens you start to come around to the way he talks about things. A good book if you're seriously interested in the topic and/or want to write an actual paper is this. Also, rapgenius.com is dope for reading along if you didn't already know.

Use this book as a tool!!!!

http://www.amazon.com/gp/product/0465003478/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0972188223&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=16T63P90Y6R2SSR5YRGX

I've begun reading this for my Poetry & Song class at university. Here's the link: http://www.amazon.com/Book-Rhymes-Poetics-Hip-Hop/dp/0465003478/ref=sr_1_1?ie=UTF8&qid=1291152308&sr=8-1

This breaks down rap's poetic elements significantly before you.