The SharePoint 2013 preview software development kit (SDK) includes documentation and code samples.
It is designed for use with three different SharePoint configurations: SharePoint Foundation 2013 Preview; SharePoint Server 2013 Preview; and SharePoint Online.
The documentation has good detailed descriptions of the technologies you use to program against SharePoint, along with documentation for the server and client object models, and step-by-step procedures for using these technologies and object models and programming with them. There’s also a best practices guide and info on setting up SharePoint to work with your custom apps.
The code samples included with the SDK are a mixture of ‘building blocks’ that you can use in your own apps, and sample apps showing how to deal with the type of customization requests you might be asked to develop apps for.
From the developer’s viewpoint, the main addition to SharePoint 2013 Preview is a Cloud App Model that lets you create apps where all the code is initially in the cloud, and optionally moved down to the client machines.
In practical coding terms, there’s a strong emphasis on moving to the new web-based "Napa" Office 365 Development Tools, though Visual Studio 2012 has a new app project template for SharePoint 2013 in addition to the SharePoint project templates that were provided in earlier versions.