I'll nominate
The Man Who Was Thursday by G.K.Chesterton.
This is probably Chesterton's best-known novel. It is a wild, surreal, Kafka-esque work which differs from Kafka in that it shows that the apparently whirling chaos actually does have a centre. About.Com describes this classic as
" . . . a book caught up in the question of whether it places itself inside or outside of the sphere of its narrative chaos, yet at the same time a book perfectly content to lose itself in that chaos and, borne by the momentum of its own energy, to keep upping the narrative and metaphysical ante until the end--no matter the cost."
"The story is told within a frame of 'real life' opening with a grim and lurid sunset in Saffron Park where the hero, a poet named Gabriel Syme, has come to a social discussion group . . . He meets another poet, an anarchist, who has an intelligent sister. The two poets argue . . . then as they leave the meeting to walk home, they fall into a wild adventure upon which hinges the fate of London."
The novel involves "sword fights and a mad chase across . . .London, . . . a love story" and blends "the format of an adventure/detective story. . . "
From
The Outline of Sanity by Alzina Stone Dale
It's a remarkable allegory loaded with remarkable ideas.
It's in the Public Domain and available right here at Mobile Read.
https://www.mobileread.com/forums/showthread.php?t=15837