The Full Nuance of the Language
I read a fair amount of erotica, some of it historical. I ran into The Horn Book: A Girl's Guide to the Knowledge of Good and Evil recently in an article or blog post and it sounded interesting. Despite its translation to English being firmly in the public domain, the only options I could find to get it involved paying money to shadowy groups who publish public domain works for ... variable amounts of money. I try to avoid paying less than scrupulous people for things that are in the public domain but sometimes it's unavoidable.
The Horn Book (at least the bits I've read so far) is a dialogue between two lovers. The more experienced man and the married woman who's having an affair with him. She asks him to teach her all the things about sex that she doesn't already know.