The Nightingale's Nest by Sarah Harrison
Left a young widow by the Great War, Pamela accepts employment as amanuensis to the Jarvises, a charmingly eccentric couple whose elegant Highgate house is a mecca for artists. She is particularly drawn to the work of a young woman, Suzannah Murchie, whose powerful portraits adorn the Jarvises' walls, and to the subject of one of the portraits, John Ashe. Ashe is a man of contradictions-- handsome, but horribly disfigured; ruthless, but charitable; wealthy, but secretive. When she agrees to work for him, Pamela is only half aware that she is entering into a pact with the devil. For Ashe has gained wealth and influence by preying on the weaknesses of others, and although Pamela keeps her distance from his activities, she cannot avoid being tainted by them.