Death of a Witch by M.C. Beaton

Returning from holiday Hamish becomes unaccountably worried - it's as if he senses a dark cloud of evil hanging over Lochdubh. He soon learns that there is a newcomer to the village, a woman called Catriona Beldame, and that the villagers have decided she is a witch.
At first Hamish is charmed by her, but is dismayed when he finds out she's been supplying dangerous potions to certain local people. No one seems willing to listen to his warnings and when she is found murdered, poor Hamish is the prime suspect. Obviously, he must solve the murder to clear his own name and bring contentment back to his beloved Lochdubh.
Praise for the Hamish Macbeth series:
'First rate... deft social comedy and wonderfully realized atmosphere.' Booklist
'It's always a special treat to return to Lochdubh.' New York Times Book Review
'The detective novels of M. C. Beaton, a master of outrageous black comedy, have reached cult status.' Anne Robinson, The Times