The Blue Hour by Paula Hawkins

Welcome to Eris: an island with only one house, one inhabitant, one way out. Unreachable from the Scottish mainland for twelve hours each day.

Once home to Vanessa: A famous artist whose notoriously unfaithful husband disappeared twenty years ago.

Now home to Grace: A solitary creature of the tides, content in her own isolation.

But when a shocking discovery is made in an art gallery far away in London, a visitor comes calling.

And the secrets of Eris threaten to emerge....