The Man Who Was Thursday: A Nightmare by G.K. Chesterton

In THE MAN WHO WAS THURSDAY, hero Gabriel Syme is recruited to a secret anti-anarchist taskforce at Scotland Yard. Syme persuades an anarchist, Lucian Gregory, to lead him to the local anarchist cell, where after a convincing speech he is elected as the cell's representative to the worldwide council of anarchists composed of seven men, each using the name of a day of the week. As Syme and his allies worm their way into the organization, he discovers that reality is not as it has been perceived.