Death of a Witch (Hamish Macbeth Mysteries) by M.C. Beaton

This is a great cozy mystery.

 Hamish Macbeth, a hulking Highlander cop and the Scottish Highlands most stubborn (and romantically challenged) bachelor. He has has refused or resigned from every promotion offered so that he can stay in his beloved village, Lochdubh, with its harsh beauty and eccentric inhabitants. But the outside world keeps intruding, the vicious police hierarchy; the effect of Strathbane, an ugly city, spreading its tentacles into the Highlands.
As this story opens Hamish returns to his home village of Lochdubh from a disappointing vacation to discover a witch stirring up trouble. To Macbeth's annoyance, the sex-starved local men have fallen under the spell of Catriona Beldame, who turns out to be a runaway bride with a shady past. Macbeth longs to prove she's selling illegal (and bogus) remedies for sexual dysfunction, and warns her to stop if she is. Macbeth gets a shock when someone murders Beldame and sets her house on fire. More murders follow in As this story opens Hamish returns to his home village of Lochdubh from a disappointing vacation to discover a witch stirring up trouble. To Macbeth's annoyance, the sex-starved local men have fallen under the spell of Catriona Beldame, who turns out to be a runaway bride with a shady past. Macbeth longs to prove she's selling illegal (and bogus) remedies for sexual dysfunction, and warns her to stop if she is. Macbeth gets a shock when someone murders Beldame and sets her house on fire. More murders follow in the village, against the usual backdrop of Macbeth trying to sort out his love life.