Fiction by Stephen King.
This book is Stephen King at his best. Now, it isn't short by any means, but each page is worth reading. I listened to the audiobook, which was superbly performed by narrator Craig Wasson, and it was over thirty hours long.
As you can easily tell from the title and the cover, the story centers on Kennedy's assassination, and I don't think it's really much of a spoiler to reveal that the protagonist goes back in time to try and save poor JFK. But the surprise to me in reading it is the detailing of all the events leading up to that main plot point. The minutiae of the journey is definitely the best part of this novel.
It was recently made into a miniseries, which I'm sure is also good, but the book is really great.