The major source of information on Horace Capron is his own autobiography including his Japanese Journal. The original typescript of these (there is no holographic copy) are at the U.S. Department of Agriculture, National Agricultural Library. The Laurel Historical Society John Brennan Library has a copy of both publications. Major manuscript collections relating to Horace Capron can be found at three institutions: The Library of Congress, Yale University Horace Capron Collection, and the U.S. Department of Agriculture, National Agricultural Library. Materials relating to the early charter of the Patuxent Manufacturing Company can be found at the Archives of Maryland.
- Horace Capron, “Memoirs of Horace Capron” 2 vols., nd, typed copy, National Agricultural Library, U.S. Department of Agriculture
- Horace Capron. Reports and Official Letters by Horace Capron, Commissioner and Advisor and His Foreign Assistants (Tokyo: 1875)
- Merritt Starr, “General Horace Capron, 1804-1884,” Journal of the Illinois State Historical Society 18 (July 1925): 259-349.
- Fumiko Fujita. American Pioneers and the Japanese Frontier: American Experts in Nineteen-Century Japan (Westport, CN: Greenwood Press, 1994)
- “Stoneman’s Raid” (MOLLUS publication of Capron’s account of the battle
- Louisa Kerwin Thiers, “An American Advisor to the Japanese Government” Journal of American History 7 (October-December 1913): 1415-25.