Before 50 Shades, before internet erotica, before plastic-wrapped volumes in the corner of the bookstore, before pretty much anything we know today, there was Anais Nin.
A lot of times people try to validate erotica by grabbing onto as many canonical authors as they can. This is why you see "Top 25 Erotic Novels Ever!" lists that look more like a collage syllabus. Maybe there are some people who can masturbate to Tropic of Cancer, but I'm not one of them, and I would be worried if I was. But Anais Nin is the real deal -- a genuine meeting between modernist poetics and dirty, filthy smut.
Some of her stories may be frustrating to those looking for exhaustive detail, but there's more than enough in suggestion and style to get your motor going. Aside from that, Nin examines sexuality in interesting ways, making it simultaneously mythic and intensely real. Her landmark story collection, Delta of Venus, moves between a set of connected characters in 1920s France and captures all of the hedonistic possibility of that era as well as the uncertainty. Nin's characters are always striving for some eternal pleasure, and always just coming up short (sound familiar?).
Get a copy of Delta of Venus if you haven't already. You don't need to stuff it under your shirt: it's pretty much the most respectable porn you can buy. You can read it with or without your pants on and enjoy it in different ways. How many books can you say that about?