The third edition is due to be published in May 2012. You can pre-order a copy, or buy the “Early Release” ebook today (you will receive the final ebook version when it is available for no extra charge).
The following section is from the book’s preface, and outlines the changes in the third edition.
What’s New in the Third Edition?
The third edition covers the 1.x (formerly 0.20) release series of Apache Hadoop, as well as the newer 0.22 and 0.23 series. With a few exceptions, which are noted in the text, all the examples in this book run against these versions. The features in each release series are described at a high-level in "Hadoop Releases" in Chapter 1.
This edition uses the new MapReduce API for most of the examples. Since the old API is still in widespread use, it continues to be discussed in the text alongside the new API, and the equivalent code using the old API can be found on the book’s website.
The major change in Hadoop 0.23 is the new MapReduce runtime, MapReduce 2, which is built on a new distributed resource management system called YARN. This edition includes new sections covering MapReduce on YARN: how it works (Chapter 6) and how to run it (Chapter 9).
There is more MapReduce material too, including development practices like packaging MapReduce jobs with Maven, setting the user’s Java classpath, and writing tests with MRUnit (all in Chapter 5); and more depth on features such as output committers, the distributed cache (both in Chapter 8), and task memory monitoring (Chapter 9). There is a new section on writing MapReduce jobs to process Avro data (Chapter 4), and on running a simple MapReduce workflow in Oozie (Chapter 5).
The chapter on HDFS (Chapter 3) now has introductions to High Availability, Federation, and the new WebHDFS and HttpFS filesystems.
The chapters on Pig, Hive, Sqoop, and ZooKeeper have all been expanded to cover the new features and changes in their latest releases.
In addition, numerous corrections and improvements have been made throughout the book.