Microsoft SharePoint 2010 SDK
It contains conceptual overviews, programming tasks, code samples.
The Microsoft SharePoint 2010 Software Development Kit (SDK) contains conceptual overviews, programming tasks, code samples, and references to guide you in developing solutions based on SharePoint 2010 products and technologies.
The MAR2011 update of the SharePoint 2010 SDK contains new code samples, updated IntelliSense XML files, and updated documentation based on customer feedback.