Resources found in the Smithsonian National Postal Museum Library are:

Bierman, Stanley M., 1986, Clarence H. Eagle: No More Buffalos, The Chronicle, Vol. 38(4): 234-239.

Kingsley, Thomas C. The Legendary Persian Rug: and other high value Civil War revenue stamps. Castenholz & Sons, Pacific Palisades, CA.1993. HJ5321.K55 1993.

Rose, Robert G., 2008, The Clarence H. Eagle Collection, Collectors Club Philatelist, Vol. 87(1): 27-29.

Scott Specialized Catalog of United States Stamps, 2006, Scott Publishing Co., Sidney, OH.

Turner, George T. Essays and Proofs of United States Internal Revenue Stamps. Bureau Issues Association, Inc. 1974. HJ5321 Z7T87X.