Black coffee by Christie Agatha (copy)

Hercule Poirot and his friend Hastings are called upon to visit the home of the famous physicist Sir Claud Amory, who has devised the formula for a new type of explosive; but they learn that he has been poisoned (in his black coffee, hence the title) the night of their arrival. Poirot is now confronted with the challenge of figuring out which of the array of other people gathered at the Amory residence is the murderer. He questions every single person that was present at the night of the murder. He then concludes his investigation with the help of an old friend from Scotland Yard.