It all seems to have begun with Ms. Monks’s great-grandmother, the one known as Mama Negie, left with nine children when Thomas died at the age of 42. She was a powerful matriarch, but she had “spells” of illness over the years, her granddaughter remembered, disappearing into the tower at her grand Cumberland home and not coming out for meals. She spent time at McLean Hospital, a psychiatric institution outside Boston. Her illness was not spoken of by the family, Ms. Monks’s mother told her. “You simply did not talk about such things in those days,” she says.

It’s not hard to glimpse her mother’s regal bearing as she says this, though the daughter, with sad eyes and a warm smile, seems both more fragile and more approachable than portraits of her mother seem to be. Silence was the expectation when Mama Negie’s granddaughter Lucy  Ms. Monks’s mother  began to have “spells,” too. Today, Lucy would have been known as “an unstable or troubled child,” her daughter says, but life on an island helped to “contain” her.

Lucy could be wild and dramatic and impetuous in that untamed space. But when she married and moved to the “real world”  specifically, Boston  and then when her husband left her for what ultimately would be three more marriages (and 11 children), Lucy “broke.” And Ms. Monks was trapped alone with her.

Her earliest memories are of nannies and chauffeurs, of summers on Crescent Island and Easter vacations on Cumberland. But then her mother became fearful, certain that Ms. Monks was filled with poison from the unpasteurized milk her husband insisted the family drink. Her parents began having screaming arguments, complete with shattering glassware.

In the years after her husband left, Lucy became a spectral shut-in, hiding behind curtains or sitting motionless for hours in oversized chairs, leaving the house only to find the company of nameless men. For a time she had Ms. Monks hospitalized, begging doctors to clear the poison from her daughter’s blood. The nannies and maids and cooks all quit, saying, Ms. Monks writes, “it’s your mother, I can’t be around her.”