The A. B. C. Murders

Hercule Poirot states in this book it is unintelligent and stupid to say a madman murders because he is mad; I love that he looks to the why. Alexander Bonaparte Cust is a complex nuanced and even more importantly, entirely believable character. Even eighty years after this was written the story holds up – it's compelling and fast-paced. I don’t like the random uses e.g. loony and lunatic but given the times (“dastardly scoundrel” is also used!) overall the Queen of Crime did a very solid job of an authentic mentally ill character.