I dscovered Josephine Tey's novels after seeing several -- including this one -- on a "100 Greatest Mystery Novels" list. I have never read anyone who can throw in a surprise ending better than Tey. This is a very good read.
Mystery novel about a stranger who enters the Ashby family posing as aheir to the family fortune. He is Brat Farrar. It seems he will get away with this deception until old secrets come to light and there is a final terrible moment when all is revealed.