Actually it comes in two ways: One is SQL and the other one is NOSQL Database.
NOSQL Database consist of: Cassandra & MongoDB
SQL Database consist of: RDBMS Membase etc
I have mentioned few books and papers here. You can refer them.
The NoSQL Ecosystem by Adam Marcus (from the book The Architecture of open source applications): http://www.aosabook.org/en/nosql.html
general thoughts and comparison http://www.thoughtworks.com/articles/nosql-comparison
technical comparison http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis
a Master's Thesis - Analysis and Classification of NoSQL Databaseshttp://scholar.googleusercontent.com/scholar?q=cache:rQlbiz6bojAJ:scholar.google.com/+comparison+of+nosql&hl=en&as_sdt=0,5&as_vis=1
Hope this helps!!
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