The Blue Hour, Paula Hawkins

For twelve hours each day, the Scottish island of Eris is cut off from the mainland. No way in. No way out.

The island's only house is home to Grace - content in her own isolation, guarding the island's past. But when a shocking discovery is made in an art gallery in London, Grace receives an unexpected visitor. And the secrets of Eris threaten to emerge.

Suddenly, this is a very dangerous time to be alone . . .