The Three Taps — Ronald A. Knox

The business of death has given insurance investigator Miles Bredon a unique outlook on life in this witty Golden Age mystery.
 
Jephthah Mottram has been given some bad news from his doctor. The very rich man has only two years left to live. But he doesn't even make it that far. While on a fishing holiday in the Midlands of England, he's found dead of an apparent suicide by gas.
 
Sadly, that would be the best-case scenario for Indescribable Insurance, which wouldn't have to pay out the benefits. To that end, the company sends out its own private detective to investigate the matter. Arriving at the Load of Mischief Inn, Miles Bredon is met by a policeman with whom he served in the war, who has his own theory about the tragedy -- and it is murderous. The two men make a friendly wager over who will prove their case, never expecting just how much greed and vanity can complicate a life -- and a death . . .