The Immoralist (1902)
by Andre Gide
It is clear to me that one of the literary trends in late 19th century/early 20th century was the development of ennui/existentialism as a theme. This proto-existentialism was classified as belonging to the "Fin de siecle" period (literally end of the century in French.) Fin de siecle is typically applied to French art, making The Immoralist a clear example of the movement, but these works were spread all over Europe. One of the most significant fin de siecle type books was Against the Grain, written by a Dutchman, Joris-Karl Huymans.
Any description of The Immoralist will contain a lead like "Considered shocking when it was published." Because so much emphasis is placed on that aspect of The Immoralist, it is worth noting that the narrator spends A LOT of time rhapsodizing about young Arab boys (he is Frenchman living in Morocco because his boo has tuberculosis.) I didn't catch an overtly sexual references to the young boys, but there is plenty of admiration of their cocoa butter skin and "long, smooth" limbs.
On another level The Immoralist is a clear step down the road to the inky depths of literary modernism- only a couple of events actually happen during the book: He moves to Morocco and he moves to Normandy. Much of the space is devoted to description of emotions, locations and people. There is no conventional plot.