Livre d'Or

It is incredibly powerful, and very edgy, and I can quite see why it was ferociously controversial in its time; I do not think it would have the deeply disturbing impact it does were it not so polishedly witty and ironic.

I also think it's really clever technically in what it does with the structure of the epistolary novel; can't recall another example of the form in which it actually mattered so much that what we see actually exists as letters, and their delivery became such an integral part of the plot.