alt Hacker News~10 years ago I had a renewed interested in perspective drawing, and was struggling with an 'exercise' that utilized curvilinear perspective & oblique angles. Anyways, I stumbled on some of Rafael's drawings and found their inclusion of the guide lines / measuring lines invaluable, but for the hell of it, I emailed him some questions. He promptly responded and then dumped images and reference material on dropbox for me - very nice guy.
Personal fav is "Dürer" that demonstrates Albrecht Dürers method of projecting a spiral.