Book Two in The Shetland Sailing Mysteries by Marsali Taylor.

When a visiting yachting couple go missing from the Shetland oil capital of Brae, sailing skipper Cass Lynch overcomes her mistrust of the land world to ask for help from her old adversary DI Gavin Macrae. He discovers a link to international art theft, and warns Cass to steer clear – but when one of her sailing pupils goes missing, she goes alone to discover the secrets of the Neolithic tomb known locally as a ‘trowie mound’ ... Ghosts, folklore and a nail-biting finale at the local show come together to make an atmospheric, fast-moving thriller.