For further reading about the Nez Perce and history surrounding the novel, I recommend the following books:

To purchase, click on the book title:

History of the Nez Perce: The Nez Perce Indians and the Opening of the Northwest by Alvin M. Josephy, Jr.

The Nez Perce and Lewis & Clark: Lewis & Clark Among the Nez Perce: Strangers in the Land of the Nimiipuu by Allen V. Pinkham

The 1877 War: 

• Chief Joseph and the Flight of the Nez Perce: The Untold Story of an American Tragedy by Kent Nerburn
• Children of Grace: The Nez Perce War of 1877 by Bruce Hampton
• Nez Perce Summer, 1877: The U.S. Army and the Nee-Me-Poo Crisis by Jerome A. Greene

Nez Perce lands and the Nez Perce trail:

• Following the Nez Perce Trail, 2nd ed: A Guide to the Nee-Me-Poo National Historic Trail with Eyewitness Accounts by Cheryl Wilfong

• Clearwater Country! The Traveler's Historical and Recreational Guide: Lewiston, Idaho - Missoula, Montana by Borg Hendrickson & Linwood Laughy

The Nez Perce and missionaries:  

• Henry Harmon Spalding by Clifford Drury
• Where Wagons Could Go: Narcissa Whitman and Eliza Spalding (diaries) edited by Clifford Drury
• The Mountains We Have Crossed: Diaries and Letters of the Oregon Mission, 1838 edited by Clifford Drury

For Children: Kaya, An American Girl: 1764/Box Set by Janet Beeler Shaw