Oooh that's a hard one because it's so different for all readers! I always bounce between books I can easily consume and practically have memorized, so for me those would be Memoirs of a Geisha, Jurassic Park, or Interview with the Vampire. And depending on how viciously I need to come out of a deeper book, I'll reread an entire series from my younger days like Animorphs or hunger games 😂