The Man Who Was Thursday by G. K. Chesterton

A philosophical thriller that follows Gabriel Syme, a detective and poet who infiltrates a secret society of anarchists, whose members are named after the days of the week. With the terrifying “Sunday” as their leader, Syme, who is named “Thursday,” discovers that the anarchists are not what they seem, and that they are all part of a bizarre plot to expose the true identity of Sunday.
A blend of adventure, comedy, and allegory that explores the nature of chaos, order, good, and evil.