Unnatural Death is a 1927 novel by Dorothy L. Sayers, her third featuring Lord Peter Wimsey.
In Unnatural Death, Lord Peter investigates the death, three years before, of an elderly lady in the last stages of cancer. There was nothing suspicious about it, and the young doctor who had been treating her had checked and re-checked everything, because he had not thought she was so close to her end, but Lord Peter thinks there may have been something fishy about it after all.