The Circular Staircase by Mary Roberts Rinehart

This is the story of how a middle-aged spinster lost her mind, deserted her domestic gods in the city, took a furnished house for the summer out of town, and found herself involved in one of those mysterious crimes that keep our newspapers and detective agencies happy and prosperous. For twenty years I had been perfectly comfortable; for twenty years I had had the window-boxes filled in the spring, the carpets lifted, the awnings put up and the furniture covered with brown linen; for as many summers I had said good-by to my friends, and, after watching their perspiring hegira, had settled down to a delicious quiet in town, where the mail comes three times a day, and the water supply does not depend on a tank on the roof.