Mr. Gryce Series by Anna Katharine Green

First published in 1897, That Affair Next Door is another fascinating study in human motivations intertwined with bits and pieces of circumstantial evidence that at first make very little sense. True to Green’s style, she calls up and explains each motivation, each piece of evidence with mathematical precision until the mystery unravels and the perpetrator is punished in a most fitting fashion.

Mr. Gryce was melancholy. He had attained that period in life when the spirits flag and enthusiasm needs a constant spur, and of late there had been a lack of special excitement, and he felt dull and superannuated... moreMr. Gryce was melancholy. He had attained that period in life when the spirits flag and enthusiasm needs a constant spur, and of late there had been a lack of special excitement, and he felt dull and superannuated. He was even contemplating resigning his position on the force and retiring to the little farm he had bought for himself in Westchester; and this in itself did not tend to cheerfulness, for he was one to whom action was a necessity and the exercise of his mental faculties more inspiring than any possible advantage which might accrue to him from their use. But he was not destined to carry out this impulse yet. For just at the height of his secret dissatisfaction there came a telephone message to Headquarters which roused the old man to something like his former vigor and gave to the close of this gray fall day an interest he had not expected to feel again in this or any other kind of day. It was sent from Carter's well- known drug store, and was to the effect that a lady had just sent a boy in from the street to say that a strange crime had been committed in --'s mansion round the corner. less

A young girl frantically summons a gentleman walking by on the street to come in and help her grandfather. Arthur Outhwaite answers her cry for help only to find himself as the last person to see her grandfather alive and left with the admonishment from the dying man to deliver a letter to someone, and to that person only.