Recommended Book
Lonely Planet Yellowstone & Grand Teton National Parks
by Bradley Mayhew and Carolyn MacCarthy
I think the best guide to Yellowstone is the official guide published by the Yellowstone Association. You won't find it in book stores though. Apparently, it's only for sale in the park and on the website shop.yellowstone.org/OFFICIAL-GUIDE.
If you'd rather order from Amazon, Lonely Planet is a solid choice.
