The September Society — Charles Finch

The sitting room looked as familiar as the back of his hand, and immediately Lenox took a liking to the young man who inhabited it. He saw several small artifacts of the missing student's life---a frayed piece of string about two feet long of the sort you might bind a package with, half of a pulpy fried tomato, which was too far from the breakfast table to have been dropped, a fountain pen, and lastly, a card which said on the front The September Society. . . .

In the small hours of the morning one fall day in 1866, a frantic widow visits detective Charles Lenox. Lady Annabelle's problem is simple: her beloved son, George, has vanished from his room at Oxford. When Lenox visits his alma mater to investigate, he discovers a series of bizarre clues, including a murdered cat and a card cryptically referring to the September Society.

Then, just as Lenox realizes that the case may be deeper than it appears, a student dies, the victim of foul play.

What could the September Society have to do with it? What specter, returned from the past, is haunting gentle Oxford? Lenox, with the support of his devoted friends in London's upper crust, must race to discover the truth before it comes searching for him, and dangerously close to home.