I nominate The Go-Between by L.P. Hartley, 1953.
From
Goodreads:
"The past is a foreign country: they do things differently there."
Summering with a fellow schoolboy on a great English estate, Leo, the hero of L. P. Hartley's finest novel, encounters a world of unimagined luxury. But when his friend's beautiful older sister enlists him as the unwitting messenger in her illicit love affair, the aftershocks will be felt for years. The inspiration for the brilliant Joseph Losey/Harold Pinter film starring Julie Christie and Alan Bates, The Go-Between is a masterpiece—a richly layered, spellbinding story about past and present, naiveté and knowledge, and the mysteries of the human heart.
I also nominate The Kingdom of This World by Alejo Carpentier, 1949.
From
Goodreads:
A few years after its liberation from French colonialist rule, Haiti experienced a period of unsurpassed brutality, horror, and superstition under the reign of the black King Henri-Christophe. Through the eyes of the ancient slave Ti-Noel, The Kingdom of This World records the destruction of the black regime--built on the same corruption and contempt for human life that brought down the French--in an orgy of voodoo, race hatred, erotomania, and fantastic grandeurs of false elegance.
The Carpentier doesn't seem to be available as an ebook, but I thought it sounded interesting enough to give a nomination as I'm fine with a pbook.
Nominations are wrapping up; I'll leave them open until I've prepared the vote.