Published in 1927, Unnatural Death is the third book in the Lord Peter Wimsey series.
Wimsey and Charles Parker are interrupted, while in a teashop, by a doctor who overhears them talking about crime. He relates a tale of how he was treating an elderly lady for cancer, whose niece insisted was much nearer than death than he felt she was. When she died suddenly, without leaving a will, the doctor insisted on an autopsy, leading to bad feeling with both the niece, Miss Whittaker, and the local community. Indeed, his actions led to him having to leave the area and begin work elsewhere. Of course, Lord Peter is immediately intrigued - how many people do 'get away with murder'? However, Parker is not convinced there is a case to answer. Presumably, as an officer of the law he had enough real work to be getting on with, but Wimsey is determined to investigate.