There are several paperback books being worked on right now (MongoDB for Web Development and The Definitive Guide to MongoDB among them), but if you cannot wait, MongoDB just got a cookbook. The best part is that the cookbook is open source and resides on GitHub.
To add your own recipes or contribute in other ways, all you have to do is fork and fix. There are already recipes for Voting with Atomic Operators and Counting Tags using map/reduce. I hope I can find the time to contribute soon (maybe hierarchical data) and really look forward to seeing this idea grow.