The publisher PACKT sent me a review copy of Google App Engine Java and GWT Application Development by my friend (via email correspondence) Daniel Guermeur and co-author Amy Unruh (thanks!) and I bought Code in the Cloud Programming Google AppEngine by Mark C. Chu-Carroll.
Both are very good books and complement each other.
Mark's book gives an interesting insite into AppEngine from someone who works at Google. He covers both Python and Java development. I relied on the Python sections of the book when I wrote a Python based AppEngine application last December. I am not much of a Python programmer but his book got me going quickly and I had few problems. He uses Google's Django support for AppEngine for the Python examples and Google Widget Toolkit (GWT) for the Java examples.
Daniel's and Amy's book is a hands-on guide to using the Eclipse IDE and GWT to develop AppEngine applications in Java. They use JDO for the book examples which is probably best for the general reader but my own preference is the lighter weight objectify-appengine datastore wrapper. The example application is neat: Connectr gathers social media and provides some aggregation services. I like that the application is interesting and contains useful code. I would particularly recommend this book if you want to use Eclipse and GWT to build AppEngine applications and you want everything you need in one tutorial and reference book.