Professional NoSQL is a great introduction book to the world of NoSQL, it goes through some key NoSQL databases and explains how to install, insert, delete and query data.
The First Part
Explains what NoSQL is and what it means to store Big Data, what are the differences between Column-Oriented Stores, Key/Value Stores, Document Databases and Graph Databases.
There are examples on how to store data in MongoDB, Redis, HBase and Cassandra, what is Thrift and how to access it with Java, Python, Ruby and PHP.
The Second Part
Explains how the data is stored, what are the similarities and differences with RDBMS, how distributed storage works, how document stores and key/value stores work.
There's an important section on eventual consistency and object versioning.
Then the author explains CRUD operations in various databases, some more advanced queries, Map Reduce and Indexes.
Lastly there's a small section on Transactions and Data Integrity.
The Third Part
Goes into some detail about using NoSQL in the cloud, like Amazon's SimpleDB and Google App Engine Data Store.
Then it goes through a more detailed explanation of MapReduce, HBase, Big Data Analysis with Hive and Joins with HiveQL, database internals, protocols and data types, indexs and how data is kept synchronized across instances.
There's a small section on how to use MySql without the SQL part, making it a sort of document store.
Lastly there's some more MapReduce and Big Data tools of the trade.
In the end of the book there's a whole appendix on how to install each database.
The book by no means covers everything there is to know about NoSQL, but it is an invaluable source for anyone wanting to understand what is NoSQL, why to use it and how to start.