Why Didn't They Ask Evans? Agatha Christie

All sorts of things happen at the Grange. Queer things. People come here to get better and they don't get better - they get worse. When Bobby Jones found a dying man in the dunes of a Welsh golf course, he stood in the shadow of his own death. But he was lucky - to have escaped being poisoned! And lucky to have the quick witted Frankie, otherwise Lady Frances Derwent, to help find the would-be murderer. Their only clues: a photograph and the dead man's final words: "Why didn't they ask Evans?"