If you enjoy the occasional book that might be labeled dark-erotica, Stockholm-syndrome, bdsm, or just plain kinky, The Story of O is a classic. 
 I first read Story of O as a teenager. I found it on a dusty back shelf of a local paperback and comic book store because its all white cover stood out in a sea of paperbacks, none of which were erotic (as far as I could tell at that young age) in garish colors. It was my first erotic fiction, unless the "stories" in Playboy counted - and I can't say they were my focus. 
 I was revolted by what was done to O ... and read every last page and word. 
 Only in this century have we learned that this classic of erotic literature was written by a female member (and there were only a few) of the French Academy of Arts (title of group by memory). It is said, and I believe, that she wrote it on a dare from her lover/publisher: that women could not write a superb erotic novel.