The Inferno (1908)
by Henri Barbusse
The Inferno (L'enfer) is a proto existentialist work about an unnamed protagonist who finds a hole in the wall of his boarding house room, allowing him to spy on his neighbors in a variety of "shocking" behaviors: adultery and homosexuality included. This might be the least popular Novel on the 1001 Books List. The Inferno only has a "stub" Wikipedia page.
Considering the racy subject matter, seems like a property ripe for a revisiting but until we get the movie version, the fact that the original is brief and an early trip to the play fields of 20th century existentialism is about all that needs to be said.