NoSQL Databases by Christof Strauch, Stuttgart Media University. (PDF, 149 pages)
An overview and introduction to NoSQL databases. According to a post on High Scalibility, Paper: NoSQL Databases – NoSQL Introduction and Overview, the paper was written between 2010-06 and 2011-02.
As High Scalibility notes, the paper is a bit dated but it remains a good general overview of the area.
It does omit graph databases entirely (except for some further reading in the bibliography). To be fair, even a summary of the work on graph databases would be at least as long as this paper, if not longer.