The Thirteenth Tale by Diane Setterfield

Vida Winter has sold more books than any other writer except the Bible but now she's elderly and sick and wants to tell the real story that she's never let interviewers know - who she really is and where she came from. She hires Margaret Lea by mail and tells her the story of her life from beginning to end, without interruption and without questions, so that it can be published. But once Margaret knows the story of the fire and what happened to the twins and gets to know Vida will she want to make the biography public?