This is a great introduction to MapReduce, Hadoop, and the HDFS. A programmer with basic Java knowledge could have most of the the code examples up and running in a few hours. That said, it is a broad topic and impossible to cover in the scope of a single book. I would have preferred more coverage of the MapReduce paradigm and briefer coverage of the Hadoop add-on projects like Pig, Hive, and ZooKeeper. Also, the book left a few gaps for me with respect to preparing input data to leverage the distributed filesystem.
All in all, a well written and very informative book. I found Data-Intensive Text Processing with MapReduce an excellent companion to this book for more detail on MapReduce.