Just the Win32 API, but it’s a thing of beauty: https://sourceforge.net/projects/win32-help-chm/files/
Edit: Dead link replaced with alternate download.
Although it is now 17 years old, the Win32.hlp file from Microsoft is
still a useful reference for those, like me, still programming using
the “flat” Windows API. It still covers 99% of what I need and is much
more convenient and faster to use than the current MSDN and Windows
SDK documentation because it is much smaller. It is a file that is
possible to add to the help menu of your IDE or favorite editor and
call up instantly, even offline.