Microsoft has released a technical review SDK for SharePoint 15 (or SharePoint 2013). This preliminary SDK is called as SharePoint 15 Technical Preview Managed Object Model Software Development Kit. You can download and dig into new SDK before next version of SharePoint released.
Regarding to the release date of Office 15 products, PJ Hough, corporate vice president of development for Microsoft’s Office Division stated that;
While the Technical Preview program is already full, everyone will have the opportunity to try the Office 15 public beta later this summer, and we’ll have more to share about the release then.