Six Years by Harlan Coben

Jake is a professor at a small college. When his father dies, he heads off to an artists' retreat in Vermont to regroup. There he meets and immediately falls in love with an artist named Nathalie. After spending the summer together, she abruptly breaks things off and announces that she's marrying an old beau named Ted--and she makes Jake promise to stay out of her life. For six years, he keeps his promise, until he sees Ted's obituary.

But when Jake goes to find Nathalie, he discovers that she has vanished and Ted's real wife has no idea what Jake is talking about. So begins the page-turner Six Years. It is definitely a thrill ride, with lots of unexpected twists and turns. A great choice if you've got a long flight coming up.