William Bosner

In the novel Murder in Mesopotamia, William Bosner is the younger brother of Louise Leidner's first husband, Frederick Bosner. William was ten or twelve years old when Frederick and Louise got married. He worshipped Frederick, and Frederick was devoted to him. William was always jealous of Louise.

When it was revealed that Louise had received threatening letters, Poirot considered it possible that William might have written the letters, and might even be one of the expedition staff. Poirot was of the opinion that in William's eyes, his brother was a patriot and a martyr for Germany, while Louise was a traitor who had sent his beloved brother to his death.

William Bosner is also mentioned in the ITV 2001 TV adapatation of the novel. For a time, Poirot considered that it might be possible that William had been operating in disguise as a member of the expedition to Tell Yarimjah and that he might be the one responsible for the threatening notes being sent to Louise Leidner. Bill Coleman, Hastings' nephew in this adaptation, was seen as a possible candidate for William Bosner.