For our July trip to Yellowstone, "Yellowstone Treasures" by Janet Chapple, was indispensible. It gives point-to-point information, with mile markers, so we could see know what we were passing at any given time. It has color maps and photos and history too. I ordered it after advice from TripAdvisor last year so I'm passing it along to others. We got 4 or 5 books, but this was the bible.