R. Austin Freeman 
As a Thief in the Night 

R. Austin Freeman was one of the best mystery writers ever and this book is one to prove it. Harold Monkhouse is a very sick man but no one including his doctor seems to know what is wrong with him. When he is found dead of arsenic poisoning, his brother demands an investigation and everyone in the household is a suspect. Who was behind this cruel death; the loving wife, Barbara; her friend, Madeline or Wallingford, the rather eccentric secretary? They all inherit something when Harold Monkhouse dies. Rupert, their loyal friend enlists the help of Dr. Thorndyke who with a lot of twists and turns solves the case. In researching the crime, Thorndyke and his charming assistant Polton do some fascinating analyses with X-rays. And the reader learns a lot about the versatility of arsenic in the hands of a creative murderer.