The Thirteenth Tale by Diane Setterfield

Opening lines:

It was November. Although it was not yet late, the sky was dark when I turned into Laundress Passage. Father had finished for the day, switched off the shop lights and closed the shutters; but so I would not come home to darkness he had left on the light over the stairs to the flat. Through the glass in the door it cast a foolscap rectangle of paleness onto the wet pavement, and it was while I was standing in that rectangle, about to turn my key in the door, that I first saw the letter. Another white rectangle, it was on the fifth step from the bottom, where I couldn’t miss it.