Brat Farrar by Josephine Tey

An impostor has a fortune to win - but he may be in danger of losing his life... 

Twenty-one-year-old Brat Farrar is an orphan, alone in the world without friends or family. So when he is offered the unexpected chance to impersonate Patrick Ashby, the long-lost heir to a vast fortune on a country estate, he agrees. Brat is the spitting image of Patrick, who disappeared years ago. At first it seems Brat can pull off this incredible deception, until he starts to realise that he is in far greater peril than he ever imagined.