The Light Between Oceans by M.L. Stedman-Used

Australia, 1926. After four harrowing years fighting on the Western Front, Tom Sherbourne returns home to take a job as the lighthouse keeper on Janus Rock, nearly half a day's journey from the coast. To this isolated island, where the supply boat comes once a season and shore leaves are granted every other year at best, Tom brings a young, bold, and loving wife, Isabel. Years later, after two miscarriages and one stillbirth, the grieving Isabel hears a baby's cries on the wind. A boat has washed up onshore carrying a dead man and a living baby.

Tom, whose records as a lighthouse keeper are meticulous and whose moral principles have withstood a horrific war, wants to report the man and infant immediately. But Isabel has taken the tiny baby to her breast. Against Tom's judgment, they claim her as their own and name her Lucy. When she is two, Tom and Isabel return to the mainland and are reminded that there are other people in the world. Their choice has devastated one of them.