Why Didn't They Ask Evans? (The Boomerang Clue)

In her Autobiography, Agatha Christie said that her adventure novels, such as The Secret Adversary were easy and fun to write.  Why Didn't They Ask Evans fits this model - it's a quick, lighthearted novel and a week after finishing it, I'd forgotten much of the plot.  Bobby Jones (not that one) and the local doctor hear a scream during a round of golf.  They find a man lying unconscious, having apparently fallen off a cliff.  Dr. Thomas goes for help while Bobby stays with the man who gains consciousness only long enough to say, "Why didn't they ask Evans?"  At the inquest, a Mrs. Cayman identifies the body as that of her brother, but Bobby can't believe that this coarse woman is the girl whose photograph the victim kept in his pocket.  He discusses this with his childhood friend Lady Frances (a/k/a Frankie) and she hatches a plot to discover who the man really is - and, perhaps discover Evans's identity as well.  As I said above, I've already forgotten the identities of both the killer and the victim, but Bobby and Frankie are lively characters and Evans's identity is an amusing twist.