Mr Reese has taken on a loaded topic and in less 200 pages he succinctly gets his major points across on that most nebulous term; Cloud Computing.
Starting in the first chapter, Mr Reese begins with his definition of cloud:
1) it must be accessible from a web browser or web service api (non proprietary)
2) 0 capital expenditure to start
3) you pay for only what you use
These simple statements provide the baseline for the rest of the book.
From here he dives right into the meat of the matter. The majority of the book details the things you, and your organization, will need to keep in mind as you move, or contemplate the cloud. Some of this is very obvious; cost of ownership, security, disaster recovery, hardware costs, backup, scaling, etc but Mr Reese pulls out the threads that make the cloud different: both in good ways and bad.
For example, a new wrinkle for cloud is what happens when your cloud provider goes out of business or has a poorly worded injunction exposing all their data (including yours) to the federal government? This is not something you worry about when you own the servers. Mr Reese elegantly explains how you can make this something you don't need to worry about even in the cloud; as long as you use some type of encryption.
Another example of where the cloud provides a potentially huge win would be in disaster recovery. Here a cloud provider provides redundancy of location and with virtual machines you should be easily able to get your system up and running again fairly quickly as long as you've taken the proper precautions (snapshots and a sane backup strategy).
Throughout the entire book, he really drills in security in the cloud. In several of the chapters, not including the security chapter, he keeps coming back to how the little things you do in your design can have a huge impact on your overall security. This is a major worry point and a barrier of entry point for many and Mr Reese spends just the right amount of time explaining how you can truly mitigate the security risks.
Another thread that runs throughout the book is scaling your application. This, to me, is one of the bread and butter wins of cloud computing. Mr Reese talks to some designs that work, and some that don't, when it comes to scaling. While all scaling talk is high level, I believe he succeeds in getting you the reader, to know what questions to ask in your next architecture meeting.
The book is a great overview and it focuses you to ask the right questions when you are dealing with cloud computing. Especially on the Amazon system. Mr Reese takes great pains to point out that yes, he is biased in talking about Amazon since that what he knows. Two appendices do talk about GoGrid and RackSpace but those read more like slick marketing glossies. And that's one of the two failings of the book. The other minor quibble is that a few times Mr Reese tries to go into detail about how something is done on the Amazon cloud (especially EC2 and S3). This is a mistake given how high level this book is. The appendix on the EC2 instructions also seem a little out of place. However these are minor quibbles.
If you are looking for a great introduction to the cloud, what it is and how to think about it, then this is the book for you. If you are looking for something to help you program, interact and learn the API for say Amazon, this is not the book for you.