If you're a developer using the Microsoft platform and want to learn Windows Azure development, Developing Applications for the Cloud on the Microsoft Windows Azure Platform (Patterns & Practices) is the book for you. It's a clear book which rides on a good practical case that covers most of the important angles. Because this is a Practice & Patterns book, it also spends quite some time teaching you the right mindset for building (multi-tenant) cloud applications.

The downside of the book is that it really assumes a good familiarity with the Microsoft .NET Framework and C#. Without that, you're not going to understand much of the cases, apart from the high-level cloud information. That said, the book starts with a good explanation of why you would want to build cloud applications, the types of scenario's that fit well, and what Windows Azure (and in more general terms Platform-as-a-Service) development means. The example case really covers most scenario's and choices people will come across, and that means it also covers all the core technologies within Windows Azure. Another great thing about the book is the many links to articles and other (free e-)books that provide deeper insight into a certain aspect or technology. Be aware that Windows Azure is a fast moving platform, with changes on a regular basis. Although most of the core concepts in this book will remain the same for a long time, it can't keep up with all the new developments. I hope new editions will follow to keep up with the changes.