MongoDB, the Definitive Guide, by Kristina Chodorow and Michael Dirolf, 2010. About 200 pages. (Here’s a link to the publisher’s site.)
This is a good introduction to MongoDB, mostly from the application developer’s point of view. I am not a MongoDB expert, but after reading this book, I felt that I understood the concepts well. The topics are clearly and logically presented. The writing and editing is top-notch, as I’ve come to expect from O’Reilly.
Read this book if you want to learn what MongoDB is, what it does, and how to use it. Don’t expect that you will learn everything there is to know about topics such as administration and tuning, although it’ll be a good start. (The MongoDB documentation is an excellent reference to continue your education in those areas.)
You might be pleasantly surprised at the lack of hype in this book. It wasn’t written by wide-eyed fanboys, and it does mention the weaknesses of MongoDB, although it understandably doesn’t spend any time bashing MongoDB for having shortcomings. I think you’ll get a balanced view of the database’s strengths and weaknesses, certainly enough to make a responsible decision about whether it’s worth investigating more deeply.
To sum up, as I wrote to the authors, “Nice book. Very well written, very clear and objective.”