Just One Look by Harlan Coben

Grace Lawson is living a happy, straightforward life in the suburbs with
her husband, Jack, and two young children. But that security is about to
come to a brutal end.

For when Grace picks up a set of holiday snaps, amongst them is one
shot that doesn't fit: a faded image of a life that Grace doesn't recognise
and of her husband as she never knew him.

Within twelve hours, Jack has disappeared, a brutal hitman is stalking
her family and the safe world she knows has been turned upside down.
And all Grace can see is that the past is coming back to haunt the
present and people are getting hurt...