Dishing the Dirt by M.C. Beaton

Agatha Raisin is convinced that Gwen Simple was involved in the murders that her son committed. Gwen, though, pleads innocence and is being counseled by local therapist Jill Davent, who starts spreading rumors about Agatha's upbringing. In response, Agatha threatens to kill her--and then finds herself the main suspect in Jill's death. As Agatha and her employees in the detective agency try to find the real killer, Agatha's own life is put in danger and other people's as well...