Ooo. O.O Shunga is good too, There is a gallery site that is really informative on the subject.
In a way it is a lost art, yet also the first early version of a "playboy" magazine that were made for samurai going out to the battle field or for the rich.
It is also the earlier form of hentai that most hentai is based off of.
Tentacle hentai is based off the story of the "fisherman's wife" and the reason the genitals were printed large and the bodies contorted for better viewing was because if they did the private parts to scale it would print as blobs of ink where the fine detail would be lost.
Still the over exaggeration of the genitals remain in today's modern hentai art.
But then at one point someone started carving the ends of the wood block instead of on the smooth plank. Giving them a much finer detail with less colour bleeding.
So do you design game art?