I have been spending much time thinking about a future where document-oriented databases are the default. Though they have their problems, I think that they are far better suited for what most people want to do than relational databases.
A long time ago, the good people at O’Reilly sent me a copy of Scaling MongoDB by Kristina Chodorow, one of the developers of MongoDB. Kristina has a total of four books at O’Reilly. This one is short, but the writing is nearly perfect. The book looks beautiful too. And no, not all O’Reilly books are that good.
If you are new to MongoDB, you should get a more complete book like MongoDB: The Definitive Guide. Scaling MongoDB is about the specific, but important issues raised by scalability. Having a separate book makes sense. Indeed, Learning the basics of MongoDB is not hard. But figuring it out sharding is a lot harder and deserves a book.