Check out:
* David Toop - Rap Attack (there's two editions out, both have great stuff
on early hip hop, particularly New York stuff...)
* Brian Cross - It's Not About a Salary (another good one on early hip hop,
focusing on West Coast stuff...)
* you might also want to try and find "Subway Art" (I forget the authors),
which is a great book documenting the graf scene in New York - hip hop is
more than music, after all...
* Sarah Thornton - Club Cultures (this is a recent book about the UK
dance/techno scene...)
Kurt