The Man Who Knew Too Much by G.K.Chesterton

Horne Fisher is extremely well connected. The plans of prime ministers, foreign ambassadors, and chancellors are matters of table conversation - usually because these people are dining with him. And when a man so well connected is also a brilliant detective, all sinister motives and Descriptions systematically unfold.
Whether it is a case of police corruption, or a war with Sweden, Horne Fisher can always solve it. But Horne Fisher is also a philosopher, and not a policeman, and the murderer is seldom punished. G. K. Chesterton, author of the Father Brown stories, here introduces another detective outside the realm of conventional law enforcement.