USGS national and regional geologic and hydrologic maps in series are in grey file cabinets in Kresge Library, and are shelved by series letter and number. Extraterrestrial geologic maps are there also.
The book, Guide to USGS Geologic and Hydrologic Maps, is a guide to these maps, and to maps in the USGS Bulletins and Professional Papers. It allows you to locate maps by quadrangle name, county and state, coordinates, or type of map. The Guide is located above the map cases in Kresge.
Good sources of maps on the Web are: