Where George Reese's Cloud Application Architectures is the theoretical what-will-happen when I host in the cloud book, Jeff Barr's is the prefect compliment. It is a very hands-on guide to using Amazon Web Services specifically, from spinning up EC2 instances, using instance store versus EBS rooted AMIs, snapshotting your volumes to backup, and even coverage of EBS with RAID.
Tto really drive home how very quickly this space is evolving, the book is two months old and already there are new services available through Amazon Web Services such as the Route 53 DNS solution. Incredible.
Still I would definitely recommend this book. Granted you can learn a lot from the AWS docs online, as they are quite good, but this book provides lots of creative examples, including a lot of PHP code using the Tarzan library to interact with AWS.
A couple caveats though. This book is very amazon-centric, so you're not going to for instance learn how to use haproxy as an alternative to the AWS loadbalancer solution, or how to host MySQL instead of simpledb. Still with the AWS logo in the corner, you're probably already expecting this. And those things considered, I thought this book was a very readable and technical introduction to the entire suite of AWS offerings.