A rich online resource for National Park Service materials is the National Park History site.

References include:

Albright, Horace M. 1985. The Birth of the National Park Service, The Founding Years. Howe Bros., Chicago.

Boslough, John and John Gattuso. 1996. America’s National Parks. Publications International, Ltd.

Huth, Hans. 1957. Nature and the American. Univ. of Nebraska Press, Lincoln

Ise, John. 1961. Our National Park Policy, a Critical History. Published for Resources for the Future, Inc. by the Johns Hopkins Press,

Leopold, Aldo. 1949. A Sand County Almanac. Oxford University Press.

Shankland, Robert. 1951. Steve Mather and the National Parks. Alfred A. Knopf, New York

Tilden, Freeman. 1955. The National Parks, What they Mean to You and Me. Alfred A. Knopf

Udall, Stewart L. 1965. The Quiet Crisis. Holt, Rinehart and Winston, New York.