The Taste of Murder — Joanna Cannan

"Literate and amusing, with exceptionally believable characters." The New Yorker.
A free-spirited widow travels to the Riviera, where she meets a lord, accepts his proposal of marriage, and returns with him to England. Bunny, now the wife of Sir Charles d'Estray, is mistress of a vast estate that's fallen into decline. To rescue the property from bankruptcy, Bunny introduces the successful but distasteful measure of accepting paying guests.
In this atmosphere of deeply resented change, a poisonous plant has become the bitter brew of murder. And as a quarrelsome cast of d'Estrays, their servants and guests, and the mystified local police wander through a maze of mutual suspicion, Bunny finds herself not only the chief suspect but also a prime candidate for murder.
"Told with a devastating detachment which is equally brutal toward the English gentry, its middle-class emulators, and upstart cockney detectives." The New York Times."