If you just want to learn the code as a whole, I'm going to have to suggest the 'offical' documentation (It's Microsoft Press, IIRC. I don't remember for sure as I never got it)
... more honestly, what part of Win32 API do you want to learn?
It's expensive, but I find it's best to get things specific to what part you want to learn. There is a DirectX introduction book, medicore but it covers Win32 basics pretty well. I'd have to find it, but let me know what you're looking for and I'll check my library.
Stolid