The major maps that we used when exploring Yellowstone were the Trails Illustrated Map series by National Geographic. The main map Trails Illustrated Map 201, covers the entire park however if you plan on doing any backcountry hiking, riding, or any type of exploring I highly suggest that you also obtain the quadrant map for the area that you are planning on visiting. The quadrant maps are much more detailed than the main map and they are as follows:

 Yellowstone Mamoth Hot Springs area, northwest quadrant, map 303,

 Tower/Canyon area, northeast quadrant, map 304,

Yellowstone Lake Area, southwest quadrant, map 305

Old Faithful Area, southeast quadrant, map 302.

The details these quadrant maps show include greater clarity of the location of each individual campsites in the backcountry, hazards you may encounter, locations of footbridges, where you will have to cross creeks, and rivers without the aid of a bridge (ford), etc.

If you plan on doing any backcountry trekking, I highly advise you to be equiped with a map of the area, knowledge of how to use the map and a compass, a GPS device if at all possible and we also enlist a personal locator beacon, specifically we use Spot, of Find Me Spot.  We do not endorse any particular products but those are the specific items we use.  Make sure you are familiar with each product and are aware of each equipments use as we are not reponsible for your missuse or inability to properly trek the backcountry.