A Handful of Ash — Marsali Taylor

Book Three in The Shetland Sailing Mysteries by Marsali Taylor

Liveaboard skipper and amateur sleuth Cass Lynch is busy at marine college in Scalloway, until one night she finds an acquaintance dead in a doorway, whose hand is smeared with peat ash. Rumours spread of a strange ritual linked to the witches once burned in Shetland's ancient capital, of horned figures abroad in the night, and townsfolk behaving strangely.

At first Cass dismisses these as mere superstition, until there's a second murder, and she begins to wonder if the devil really does walk in Scalloway …