Philo Gubb: Correspondence-School Detective (1913)

Humorous children's detective novel, first published in 1918. According to Wikipedia: "Ellis Parker Butler (December 5, 1869 – September 13, 1937) was an American author. Butler was born in Muscatine, Iowa. He was the author of more than 30 books and more than 2,000 stories and essays, and is most famous for his short story "Pigs is Pigs", in which a bureaucratic stationmaster insists on levying the livestock rate for a shipment of two pet guinea pigs, which soon start proliferating geometrically."