The following publications contain information about John Ryan:
Article: "John Ryan: His Military Campaigns Rival Custer's" by Sandy Barnard • Among 7th Cavalrymen, only one man's military experiences rival Custer's own. That's Company M's First Sergeant John Ryan, whose life and career during the Civil War and on the plains are reviewed by Sandy Barnard. • Greasy Grass, Journal of the Custer Battlefield Historical & Museum Association, Vol. 8, May 1992.
Article: "A 7th Cavalryman Speaks Out" by Sandy Barnard • First Sgt. John Ryan proved the most prolific writer among 7th Cavalry enlisted men. One of his letters was sent to E. A. Brininstool in March 1926. The letter was published in February 1927 in Frontier Times. • Greasy Grass, Journal of the Custer Battlefield Historical & Museum Association, Vol. 21, May 2005.