Just finished reading of Hadoop: The Definitive Guide - this is very good book, it allows to learn about Hadoop itself, development in MapReduce style, and also get information about Hadoop-related projects - Pig, HBase, ZooKeeper, Cascading and other. This books also contains number of hints on proper organization of programs for Hadoop, and also interesting case-studies from companies, who are using Hadoop in production.
Next book in reading queue is Data-Intensive Text Processing with MapReduce (it's also available for free as last draft version). This book describes how Hadoop (and MapReduce) could be used for analysis of text data in different tasks. But to read it, you need to know about Hadoop and development for it.