This is a really great website hosting tons of the great, old erotic novels published in the sixties and seventies. Obviously it's a mixed bag, with some really amazing stuff and some really terrible stuff, but you're likely to find something that strikes your fancy there. A lot of the material from back then focused on taboo subjects like incest and bestiality, and you can probably tell the central focus from the title.
http://neatopotato.net/xnovel/By the way, there's no fisting focused books on there that I'm aware of, but I'd be shocked if it didn't appear in a few of the stories.
By the way, some of my favorites are the "Adult Classic Series" books from Liverpool Library Press. Ahhhh.... Fond memories of dog eared, stained paperbacks with those tacky green covers and the vibrator ads in the back.
