Agatha Raisin: Dishing the Dirt: Agatha Raisin Series, Book 26 (Unabridged)
A therapist moved into the village of Carsely, and Agatha Raisin hated her. Not only was this therapist, Jill Davent, romancing Agatha's ex-husband, but she had dug up details of Agatha's slum background.
Added to that, Jill was counselling a woman called Gwen Simple from Winter Parva, and Agatha firmly believed Gwen to have assisted her son in some grisly murders, although she has no proof she had done so.
A resentment is different from a dislike and needs to be shared, so as the friendship between James and Jill grows stronger, the more Agatha does to try to find out all she can about her. When Jill is found strangled to death in her office two days later, Agatha finds herself under suspicion - and must fight to clear her name.