11/22/63 by Stephen King

English teacher Jake Epping discovers a portal in a diner's storeroom that leads to 1958, and with it a chance to stop the assassination of President Kennedy. Living under a new name in small-town America, he builds a life, falls in love, and learns that the past fights back when you try to change it.

Part thriller, part love story, part time-travel adventure, this is Stephen King at his most absorbing and big-hearted.