The Light Between Oceans by M. L. Stedman

After experiencing the horrors of The Western Front, Tom moves to Janus Island to work as a lighthouse keeper. He hopes that life on the remote island with his wife Izzy will bring him peace following the war.
The two of them are happy together, although frustrations arise the longer they try for a child that never comes.
But then, one day, a boat washes up on the shore of their little island. In it, lies a dead man - and a crying baby.
Faced with an impossible decision, Tom and Izzy do what they think is right and follow their hearts. What happens next will break yours.