The Last Girl to Die - Helen Fields

In search of a new life, the family of seventeen-year-old Adriana Clarke moves to the ancient, storm-battered island of Mall, far off the coast of Scotland. Then she disappears. Facing hostile locals and an indifferent police force, her desperate parents turn to private detective Sèdie Levesque.
Sèdie is the best at what she does. But when she finds Adriana’s body in a cave on the cliffside, with a crown of seaweed carefully placed on her head, she realizes she’s dealing with something she’s never encountered before.
The deeper she digs into the island’s secrets, the closer danger creeps – and the more urgent her hunt for the killer becomes. Because what if Adriana isn’t the last girl to die?