Epistolary Novel; Satire and Parody
Les Liaisons Dangereuses is an epistolary novel: it's made up of letters written by the main characters in the story to one another. We read a lot of "my dear" and "good-bye" and "I have the honor to be, etc."
Les Liaisons Dangereuses has its moments of black humor, but unless you sympathize with the two nihilistic lead protagonists (antagonists?), the novel is pretty depressing. Nobody's happy by the end, not the vicious, not the virtuous.
What keeps the book from being a complete downer is enjoying the over-the-top villainy of the Marquise de Merteuil and the Vicomte de Valmont. Their letters are dripping with hilarious, if nasty, descriptions of other people. If these two figures weren't such well-developed characters, they'd be caricatures.