Gosh, thinking about devil novels with that dark fantasy edge really brings 'The Invisible Life of Addie LaRue' to mind. It’s not a classic devil deal, but the ancient, nameless entity she bargains with fits perfectly—there's this gorgeous, melancholy vibe that mixes historical fantasy with a truly cursed existence. The darkness comes from the emotional cost, not just gore, which I crave sometimes.
For a more visceral, hellish landscape, check out 'The Demon's Librarian'. It’s a bit more niche, but the world-building has demons fighting for dominion in a modern city, and the protagonist gets tangled in their politics. It leans into occult lore and has that gritty, urban decay feel. If you want something where the devil isn't just a metaphor but a faction warring over souls, that's a solid pick.