Mayor's Wife, The by Anna Katharine Green (1864 - 1935)

Miss Saunders is out for an adventure. One, which is full of secrets, hints, and half-lies. One, which will require all of her wits. She is to be the companion to the Mayor's wife. The Lady is unhappy, and the reason for her grave unhappiness is more serious than you think. (Summary by Stav Nisser)