Can you recommend some great book to learn how to make app with web services for the Google app engine?
I found Programming Google App Engine by Dan Sanderson very immersive and thoughtful. His treatment of the topic is in-depth and goes beyond hello world programs. I recommend this book for all those who want to learn GAE in-depth. This book is very GAE specific and hence concentrates on both Java and Phython API's. You fill find very little content on GWT in this book.
If you are a GWT beginner there is Robert Hanson's GWT in Action.
In addition to the 2 books Google has a number of good articles/dev videos on the respective SDK sites, which will help you to get started. It all depends on how much time you have in your hands, If you are short of time, learn GWT from Google's tutorials and totally skip the GWT book, and concentrate on mastering GAE and Datastore. You can always learn GWT on the way.
I do not know of any book which covers GAE + GWT in depth.