The Golden Unicorn — Phyllis A. Whitney

Don't try to find out who you are, Courtney. You may uncover horrors you're better off not knowing. Be satisfied with the loving parents who raised you. Let the door stay closed....
On the southern shore of Long Island, in a foreboding East Hampton mansion, a violent encounter is in the making. Courtney Marsh has arrived at one of the island's great ancestral estates in search of a truth that has eluded her a lifetime: the identity of her own mother and father.
The only clue she has to her past is a pendant she has always worn-a pendant bearing a golden unicorn-long an insignia of the strong-willed Rhodes family, one of East Hampton's most prominent, yet most reclusive, clans.
But from the moment of her arrival there, Courtney's reception is quite different from any that a long-lost daughter might expect. For Courtney Marsh's fight to discover her heritage was unearthing the hushed-up facts behind a murder committed twenty-five years before...a very private scandal, the unveiling of which could not help but force the murderer's hand again...