Man Who Was Thursday

Often referred to as a metaphysical thriller, G.K. Chesterton's brilliant 1908 novella The Man Who Was Thursday - A Nightmare is a tour-de-force of suspense-writing.
Newly recruited Scotland Yard detective Gabriel Syme infiltrates a dangerous underworld anarchist group with the help of a poet he befriends, named Lucian Gregory. The taut adventure that ensues is part spy narrative, part dystopian novel and part Christian allegory.

Chesterton's unconventional masterpiece has been described as "one of the hidden hinges of twentieth-century writing, the place where, before our eyes, the nonsense-fantastical tradition of Lewis Carroll and Edward Lear pivots and becomes the nightmare-fantastical tradition of Kafka and Borges."