The Man Who Was Thursday by G. K. Chesterton
Gabriel Syme is dispatched by Scotland Yard on a secret mission to infiltrate the Central Anarchist Council - an organization plotting to bring down the existing social order. The seven members of the group are named after days of the week, with the mysterious Sunday - who calls himself 'the Sabbath and the peace of God' - as their leader and mastermind. Having successfully infiltrated their ranks, Syme himself becomes known as 'Thursday'. But he soon finds himself in a surreal waking nightmare, in which the lines between freedom and order, fact and fiction, become irrevocably blurred.
Written in 1908, and drawing heavily on contemporary fears of anarchist conspiracies and bomb plots, The Man Who Was Thursday remains uncannily relevant. It is a fascinating mystery, a spellbinding allegory and an entirely chilling classic of crime fiction.