Death of a Dishonorable Gentleman, by Tessa Arlen

Lady Monfort's summer ball is even more memorable than usual when her unpleasant (and Dishonorable) nephew is murdered. When her son comes under suspicion, she enlists the help of her housekeeper, Mrs. Jackson, to conduct an upstairs/downstairs investigation and clear him.  The two woman are so uncomfortable in this nontraditional relationship, it feels quite possible. Very enjoyable.