The Devil and Drusilla — Paula Marshall

The cynical Earl of Devenish, often most deserving of his nickname Devil, little expected to be so powerfully drawn to his neighbor Mrs. Drusilla Faulkner. Quiet and demure in public, in private she showed a wit and strength of will to match his own.

But her husband's untimely death was part of the puzzle he was investigating,

Hero: Henry Devenish (Devil), Earl of Devenish
Heroine: Drusilla Faulkner