11/22/63 explores the idea of what might have happened had someone gained the ability to travel back in time and change a few things. The main goal, of course, is for the main character, Jack Epping, to go back and prevent the assassination of President John F. Kennedy. It all starts with Jack's not-quite-friend, Al. Al owns a diner that houses a secret: a "rabbit hole" into the past.
Each time a person travels into the rabbit hole and back, no matter how long he stays in the past, no more than two minutes in the present passes. Every change made resets. Almost. King, in my opinion, does a great job of coming up with ideas that seem plausible, were time travel possible. A reoccurring theme in the book is the past doesn't like to be changed. The past dislikes change so much that it fights back.
There is even the idea that, no matter how much you want to change certain things, they wind up happening anyway but not always for the same reasons. Certain events in history are preordained. Not only does the past hamper Jack's attempts at changing it, but it often finds ways to undermine things he manages to change anyway. There's a feeling, while reading along on the whirlwind story line, that if he accomplishes what he sets out to do, saving J.F.K., the future is not going to be exactly how Al thinks it's going to be.
The future Jack comes back to when he finally returns to the present is definitely a bit of a surprise. King hasn't moved away from horror. Far from it. Rather, he's figured out that subtle can be just as scary, if not more so, than the in-your-face approach he focused on in his early works. 11/22/63 is, overall, more science fiction than it is horror, but it will appeal to even wider audiences than these. It's definitely a recommended read.