The Seventeenth Stair (by Barbara Paul)

After spending most of her life at the Seminary for Daughters of Gentlefolk in Kent, Rosella Eastwood, at the age of seventeen, was now to enter a new world. She was to meet her guardian, Andre Cadel, in Paris and from there - a new life. Rosella could never have imagined the turn her life would take. She was surrounded by the de Louismonts, the family who resented her very existence. Her home, although a charming chateau in the Loire valley, was a house of hidden rooms, of eerie sounds and of mysterious accidents.