Hadoop: The Definitive Guide
http://www.hadoopbook.com/
Data-Intensive Text Processing with MapReduce
http://www.umiacs.umd.edu/~jimmylin/book.html
It focuses on programming in MapReduce and not specifically on Hadoop.
Hadoop: The Definitive Guide is best book to start with. I found Jason Venner's book to have a good amount of coverage that doesn't overlap with HTDG. Specifically if you are going to use the streaming apis.