The Master of Blacktower by Barbara Michaels

In an isolated mansion known as the Black Tower of Dunnoch, Damaris Gordon meets her benefactor, Gavin Hamilton, a cruel and bitter man tortured by disfigurement. In spite of a past that hints at his involvement in his wife's death, Damaris falls victim to an overpowering love that will either lead to the truth--or push her over the edge. - FF

Yep, it's the same lady. She's Elizabeth Peters, too. As Barbara Michaels she writes gothic tales and supernatural thrillers. - Mattie Rae