A free guide is available on MSDN patterns & practices section, demonstrating how you can create from scratch a multi-tenant, software as a service (SaaS) application to run in the cloud.
This guide is the third release of the second volume in a series about Windows Azure. It demonstrates how you can create from scratch a multitenant, Software as a Service (SaaS) application to run in the cloud by using the latest versions of the Windows Azure tools and the increasing range of capabilities of Windows Azure.
The guide is intended for any architect, developer, or information technology (IT) professional who designs, builds, or operates applications and services that are appropriate for the cloud. It is primarily written for people who work with Windows-based systems. You should be familiar with the Microsoft .NET Framework, Microsoft Visual Studio, ASP.NET, SQL Server and Microsoft Visual C#.
You can check ou the guide at this MSDN page.