Sara knew that her life was in danger - bu she threatened by a ghost or a flesh-and-blood murderer?

While Sara's friend Linda was working in a French chateau as governess to a small boy, Luc, she wrote of a marvelous man she had fallen in love with. Then Linda died in a suspicious circumstances. Sara resolved to find out more about the mystery and went to the the chateau in her friends place. Who was the man of whom Linda had written so passionately? Was it Luc's father, Giles, his brother Tobie, or his disturbing cousin Romain?