Death of a Witch by M.C. Beaton

Policeman Hamish Macbeth comes home from a vacation in Spain to find that the men in the village have been visiting newcomer Catriona Beldame late at night. After going to Catriona's home, he discovers that she's been supplying the men with potions for sexual dysfunction. Hamish warns Catriona to stop, since the men are becoming ill. She refuses and soon after, she's found dead in her cottage. Hamish, at first, is the prime suspect because he was overheard saying that he wanted to kill her. But then, several more people are murdered and Hamish has his hands full, not knowing if there's a serial killer on the loose. Hamish is also busy romantically as there's a new forensics policewoman, Lesley Seaton. Will he be able to bring the cases to a close and also finally find the love he's been searching for?