A new copy of the SharePoint Products and Technologies 2003 Software Development Kit (SDK) has appeared and is available for download.
This SDK contains conceptual overviews, programming tasks, samples, and references to guide you in developing solutions based on Microsoft SharePoint Products and Technologies.
It also includes new documentation for the Web Part Page Services Component (WPSC), which is a client-side component that adds dynamic capabilities to your Web Part Page by providing Web Part discovery, notification, and state management services used by Web Parts; and for Windows SharePoint Services RPC methods.