The Witches' Tree by M.C. Beaton

When Margaret Darby is found dead, hanging from a tree, in the village of Sumpton Harcourt, gossip abounds that a local witches' coven is responsible. Private detective Agatha Raisin is hired by Sir Edward Chumble to find the murderer. Agatha and her employees at the agency, along with friend, Charles Fraith, delve into the lives of the residents of the small village and try to determine if there are actually people around practicing witchcraft. Agatha, as usual, puts herself in harm's way on her journey to find the culprit before the police do.