Encounters at the Heart of the World, a history/anthropology of the Mandan nation of the American plains.
"In this extraordinary book, Elizabeth A. Fenn retrieves their history by piecing together important new discoveries in archaeology, anthropology, geology, climatology, epidemiology, and nutritional science. Her boldly original interpretation of these diverse research findings offers us a new perspective on early American history, a new interpretation of the American past."The biography prize went to The Pope and Mussolini by David Kertzer. The nonfiction prize (yeah, it's complicated) went to New Yorker writer Elizabeth Kolbert for The Sixth Extinction, her history of the ongoing mass extinction of species around the world.
Update, April 27: A different view on extinction history: Stewart Brand says not so.