The Blue Hour by Paula Hawkins

In the dead of night, Grace wakes . . . The Scottish island of Eris is largely unreachable from the mainland.

Cut off by the tide for twelve hours each day. 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 . . .