In my adolescence I found a book in one of the drawers of my dad's desk, which was about various sexually related things. One chapter was about S & M, spanking etc.
I was fascinated and aroused by a supposedly true story of a teenage boy who was once sent to stay with a woman who'd taken in a few teenagers. He once heard some noise, and through a keyhole observed the woman thrashing a girl, who was naked at least beneath her waist.
A day or two later the woman walked into his room when he was wearing just a shirt, told him to lay down, and thrashed him, I think perhaps with a birch.
I'm sure there were details re. erections and whatever, but it's such a long time since I last read it.
Weirdly, one day when I took out the book the chapter that had been of most interest to me had been cut out. Not ripped out, but cut around, so that only the outer words on the pages were visible.