The Apple iTunes SDK (“iTunes COM SDK for Windows”) is astonishingly difficult to find. It can be found on the Dowloads & ADC Program Assets (connect.apple.com) page. In the “Downloads” box on the right, click on “Developer Tools,” then search the page for “iTunes COM SDK for Windows.”
The irritating thing is that there seems to be no direct path from the Apple Developer’s Center to this “Downloads & ADC Program Assets” page.