Philosophy in the Bedroom: "The 120 Days of Sodom" by Marquis de Sade

(Original Review, 2008)
Personally,I think the way to understand De Sade is as a global pioneer in the art of trolling. His actual sexual acts were fairly tame in the broad scheme of things - 15 year-old servants, but people were getting married at that age in his time. As far as history records, his actual practical sexual tastes didn't extend much further than a little light flagellation and buggery. What he really loved was to shock and upset people with what he wrote, and he was extraordinarily good at it. In 'Philosophy in the Bedroom', he extends the theory that God must want us to have anal sex, or he wouldn't have made our arseholes so deliciously tight.
If you're into stuff like this, you can read the full review.