The Franchise Affair by Josephine Tey
Marion Sharpe and her mother seem an unlikely duo to be found on the wrong side of the law. Quiet and ordinary, they have led a peaceful and unremarkable life at their country home, The Franchise. Unremarkable that is, until the police turn up with a demure young woman on their doorstep. Not only does Betty Kane accuse them of kidnap and abuse, she can back up her claim with a detailed description of the attic room in which she was kept, right down to the crack in its round window. But there's something about Betty Kane's story that doesn't quite add up. Inspector Alan Grant of Scotland Yard is stumped. It takes Robert Blair, solicitor turned amateur detective, to solve the mystery that lies at the heart of The Franchise Affair.