The Big Four (1927) by Agatha Christie

In The Big Four, Hercule Poirot takes on a puzzling series of murders that seem unrelated at first glance. But as always, the meticulous Belgian detective spots the one subtle clue tying them all together: the recurring presence of the number four. Determined to uncover its meaning, Poirot soon discovers the existence of a powerful international crime syndicate known only as “The Big Four.” Joined once again by his loyal friend Captain Hastings, Poirot embarks on a chase across Europe, tracking a network that always seems to stay one step ahead. It’s a race against time to expose a conspiracy that reaches far deeper than anyone imagined.