APA (7th ed.) Citation

National Geographic Society. (2001). National Geographic guide to the national parks of the United States (3. ed., completely updated.).

Chicago Style (17th ed.) Citation

National Geographic Society. National Geographic Guide to the National Parks of the United States. 3. ed., completely updated. Washington, DC, 2001.

MLA (9th ed.) Citation

National Geographic Society. National Geographic Guide to the National Parks of the United States. 3. ed., completely updated. 2001.