Azure In Action by Brian Prince and Chris Hay has something for everyone. It provides a good overview of the use cases for Windows Azure and a high-level overview of the Azure architecture, which is useful for those new to the platform. It also provides many in-depth examples of Azure features, such as web roles, worker roles, and storage options.
The book also benefits from the light-hearted style of Prince and Ray, who are as entertaining in print as they are in person.
The only downside is that newer Azure features are not covered in this book and Microsoft is adding new features at a startling rate. As far as I know, no updated edition is in the works to cover these new features.
Still, the book remains relevant because of its focus on the uses of cloud computing and on the still-relevant core features.
If you are new to Windows Azure, this book is a good starting point.