In my role as a documentation writer at Microsoft I recently completed and posted a new release of documentation for developing interactive HD DVD content, also known as iHD. You can find the documentation at: http://msdn.microsoft.com/library/en-us/HDDVDATK/htm/hddvdprogrammingguide.asp.

An interesting and informal resource for authoring iHD content can be found on MSDN at Peter Torr's blog.