Very Old Money - Stanley Ellin

Two servants discover family skeletons behind the closed doors of a forbidding Manhattan mansion in this mystery by a three-time Edgar Award–winning author.
In dire financial straits, young couple Mike and Amy Lloyd—a former cab driver and a New York prep-school teacher, respectively—have signed away their independence to become live-in servants for one of the city's wealthiest and most private families.
At first, the Durie home, a cavernous Gilded Age palazzo off Fifth Avenue, is a maze of intimidation: sixteen other employees, eight Duries in residence, forbidden rooms, and an exact and unbreakable set of rules. For Amy, personal secretary to the aged and blind Miss Margaret, that includes never broaching the subject of her employer's "condition" or the tragic accident that caused it. On the other hand, Mike, an aspiring writer, is already taking notes for a Durie-inspired novel. A modern gothic, he's guessing—part Rebecca, part Psycho. Most of the plot, he'll soon discover, won't require much imagining.
But Amy, bound to the servitude of the matriarch—a woman cut off from the world for fifty years—is growing more curious and unnerved by Miss Margaret's demands: the sudden trips to the Plaza hotel, the mysterious bank transactions, and an extended invitation to a stranger for a private dinner. By the time Amy realizes the truth—that she and her husband have been enlisted as unwitting accomplices in a subtly played series of moves that could lead to something rather unspeakable—it could be too late.