Tajemnica Bladego Konia by Agatha Christie

After learning about a sequence of mysterious deaths tied to a run-down public house with a sinister reputation, a man sets out to investigate and gradually strips away rumor and superstition to reveal that a small circle of women—rumored to practice witchcraft—are in fact part of a coldly rational scheme using modern poisons; careful detective work uncovers the conspiracy, motives and methods behind the killings and brings the perpetrators to light.