There's a God on the Mic, subtitled "The True 50 Greatest MCs", is MC Kool Mo Dee's work on the greatest MCs of all time, explaining who is great and why. Generally, I don't think these lists are very valid, but I bought this book because there is a lack of books written about hip-hop, and KMD's reputation is strong.
This book stands out as a worthwhile "top 100 list" book, and also as an inside look at what makes a good MC. KMD has a very technical, 17 point system, for rating MCs, and although some people may not agree with all of his judgements, they are actual judgements, and not just opinions. KMD also has his own biases: he favors East Coast, Old School and political/consciousness artists, and purposely overlooks rappers who were most famous for their work in groups. (He is planning on writing a seperate book about the 50 greatest rap groups). Also, notable, given the reputation for self-aggrandizement and feuding in hip-hop, is that Kool Mo Dee gave a fairly balanced presentation, placing himself at only #5 on the list, and giving arch-rival LL Cool J the #7 slot. Also, despite his own leanings away from violence and raw sexuality, he gives these rappers their due, even while explaining that it is not his personal taste.
As a personal commentary, when I was a little younger than I am now, I took rap music, and its potential to change the society around me, very seriously. Over the years, I have lost a lot of that faith, perhaps because I have changed or perhaps because hip-hop has changed. Reading this book, and seeing that Kool Moe Dee looks upon MCing very seriously, helped recreate the earlier excitement of hip-hop music for me.
For serious students of hip-hop, this book is a very key read.