
A brief introduction to the class of non-relational databases known as "NoSQL." The book covers core concepts as well as implementation issues and use cases.
My favorite. Not an easy read, it took me weeks to complete it.
https://www.amazon.com/Date-Database-Writings-2000-2006-Chri...
Relatively unknown but great book if you want to read the thoughts of a DB heavyweight Christopher Date who collaborated with Edgar Codd.
https://www.amazon.com/NoSQL-Distilled-Emerging-Polyglot-Per...
The best book to start exploring the land outside the relational world. An easy read, can be completed in two to three days.
The only thing I've read so far is Fowler's "NoSQL Distilled". It's not exactly a hardcore text but it does give a good overview of the different types of databases, and the tradeoffs they make in terms of consistency/availability.