During our first stay in a hydrothermal zone in Yellowstone we discovered the simple but very informative trail guides. Since then they have accompanied us through our stay in Yellowstone National Park. For only 1,- Dollar you get a simple map (which is very easy to read and follow because of the everywhere available plank paths) and good background information to most geysers, hot springs and pools. They are published by Yellowstone Forever, but I haven’t found them anywhere in the online trade.
We would recommend them to any visitor of the park. Our hiking maps bought in advance were mostly used for preparation and especially for estimating the length of a hike, but they were usually in a backpack during the hike. The small trail guide was always easily accessible and stowed away in one of the backpack side pockets!