I've found this book to be quite solid so far: http://shop.oreilly.com/product/0636920032175.do

I'm about a quarter to half of the way through and it's been interesting and quite thorough even though it's still a "beta" book. The content is a little high level so some familiarity with distributed systems principles is useful but the text is very approachable and easy to understand (so far).

I got it after seeing several recommendations in other HN threads so I'm not the only person that has found it useful.