- The Great Lakes: An Environmental History (1986)
- Forest Primeval: The Natural History of an Ancient Forest (1993)
- The Old Northwest: A History (1997)
- America's National Parks: A History (2001)
- Empire of the Summer Moon: Quanah Parker and the Rise and Fall of the Comanches, the most powerful Indian tribe in American history (2010)
- The River That Made Seattle: A Story of Reinvention (2013)
- The Land Remembers: The Story of the Great Lakes Forests (2018)
- Tales from the Dells: A History of Wisconsin Dells (2020)
Westhoff's work has been praised for its historical accuracy, engaging writing style, and environmental sensitivity. He has won numerous awards for his work, including the John Burroughs Medal, the National Outdoor Book Award, and the Washington State Book Award.
In addition to his books, Westhoff has also written articles for magazines such as Audubon, National Geographic, and The New York Times. He has also appeared on television and radio programs to discuss his work.
Westhoff is a graduate of the University of Wisconsin–Madison. He lives in Madison, Wisconsin, with his wife and two children.