Christof Strauch, from Stuttgart Media University, has written an incredible 120+ page paper titled NoSQL Databases as an introduction and overview to NoSQL databases. The paper aims at giving a systematic and thorough introduction and overview of the NoSQL field by assembling information dispersed among blogs, wikis and scientific papers.
Document reviewing CouchDB and MongoDB as the two major representatives of this class of NoSQL databases, takes a look at column-stores by discussing Google’s Bigtable, Hypertable and HBase, as well as Apache Cassandra.
Highly recommended!