Allen, John Logan, Lewis and Clark and the Image of the American Northwest, Dover Publications, 1991.

This reissue of the 1975 book Passage Through the Garden deals with Lewis and Clark and the concept of North American geography at the time of the expedition.

Appelman, Roy, Historic Places Associated with Their Transcontinental Exploration, National Park Service, 1975.

The first half of this book presents a historic overview of the expedition, and the second half talks about the sites along the trail as they exist today.

Dillon, Richard, Meriwether Lewis: A Biography, National Park Service, 1975. This is an early biography of Lewis.

Duncan, Dayton, Lewis and Clark: An Illustrated History, Knopf, 1997.

This companion piece to the Ken Burns' documentary Lewis and Clark: The Journey of the Corps of Discovery includes contributions by Ambrose and other noted writers.

Jackson, Donald, Thomas Jefferson and the Stony Mountains:

Exploring the West from Monticello...