The Case of the Murdered Mathematician, by Harry Stephen Keeler (epub/Kindle/pdf)

Quiribus Brown, probably the only 7-and-a-half-foot-tall mathematical detective in the Midwest, has got to solve the mystery of who killed Professor Munstergale, or he’ll never get his rightful inheritance. Luckily, Munstergale lived long enough to draw on a blackboard two diagrams, one pointing to his killer, and the other pointing to where he knew the killer from. There are two theories, both involving prominent citizens as suspects, and Inspector Clarvoe’s job is depending on Quiribus’ deciding which theory—or neither—is correct.