I must not tell lies, the coda, and the sequel
This was a very good story for a lot of reasons. It's called I must not tell lies. You can read it here. The links to the coda and the sequel are included.
So, if you like spanking stories, you'll like this one. It's not the typical spanking story, so I don't want to qualify it as that. More, I'd call it a beating story, because, other than Dean's sympathy, there's very little comfort after the beating. More, it explores each character's point of view, and slowly unfolds why things happened the way they did. John accuses Sam of loosing an important book, and Sam denys it, in the way that he does, "What? I just spent HOURS translating, you idiot, you think I would just go and LOOSE all that?" Well, he doesn't exactly say that, but his reply sends John over the edge, and it gets complicated from there.