Well, there is the Wikipedia Distributed Hash Table page as an introduction. I don't think you're likely to find anything specific to how BitTorrent would use it, but there are known implementations of a DHT: Pastry and Chord.
I've read through the academic paper describing Chord and it gave me a pretty good idea of how the general DHT algorithm worked. And a slideshow from Carnegie Mellon starts talking about DHT on slide 37.
Of course, if you are looking for precisely how BitTorrent does it, you'd probably need to grab their reference implementation and look through the code.