How to Choose a Shard Key: The Card Game Kristina Chodorow’s highly entertaining post on how to evaluate sharding strategies.
I mention this because sharding is likely to become an issue as top map applications grow in size. Evaluating strategies before significant development time and effort are invested is always a good idea.
I also have a weakness for clever explanations that capture the essence of a complex problem in an accessible form.
If you are at all interested in MongoDB, see the rest of her blog entries.
Or, MongoDB: The Definitive Guide by Kristina and Michael Dirolf.