Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Multiple First Person Narrators
Since the novel consists entirely of letters between the main characters, we have multiple first-person narrators and multiple perspectives and biases. We also get multiple purposes and audiences, even from the same character. When Valmont's writing to Madame de Tourvel, he's being seductive and deceitful, not revealing his true self. When he's writing to the Marquise, he's usually bragging or ranting. We can't entirely trust what he writes in either case. To get a sense of the real truth, we have to look at all of the letters together. That goes for everyone's correspondence. Even then, we're getting highly subjective versions of the truth.