I'd have rated this a 4.5 if I could have.
This is an excellent book covering a very new subject matter. My only major complaint is that it seems a little rushed - I've found several typos, and even one section where a couple of lines of (important for that section) code are missing. (I figured out what was missing as I'm sure most people will.)
Also, the book is out of date. However, that is not the fault of the author or the publisher! It is that Amazon's service changes so quickly. The author and the publisher have made every attempt to mention the most recent changes to the service as of the time of writing, including pointing to places on the web to find out more information.
The material it covers is spot on. It goes through the different services that Amazon offers - including their storage, elastic computing, payment systems, and database systems. It clearly explains the disadvantages and advantages of each system, and provides -useful- code examples (in ruby) of how one can take advantage of the services Amazon provides. (There are examples in other languages, like Python, that the author makes available on the book's website.) Each section is devoted to a service for the most part, and the book is very readable.
As I said, I'd have rated this book a 4.5 if I could have. Outside of the errors due to rushing, it's quite useful and quite informative. The code is easy to follow, and I've found it very handy for working with the Amazon Web Services.