This is really frustrating as I'm pretty sure that when I read this book, I posted something about it on this very forum.
Anyway: there's this dude who keeps reliving the same few years from his life, and each time round he's able to profit from his experiences. It so happens that the time period includes November 1963, and of course he tries to prevent the Kennedy assassination, as you would. He manages to get Oswald arrested by the FBI - then is stunned to hear that JFK has instead been shot dead by some other rootless drifter. Of course this book came out long before Stephen King wrote a "counterfactual" novel with a similar premise.