This 19 page essay by Keyes outlines what he considers to be the best way to teach students to read moderately difficult German in the second year of instruction.
This article, written in 1903 in the Proceedings of the American Philological Association, discusses the history of the auxiliary verb in the German language.
This letter from U.S. House Representative G.N. Haugen, acknowledges the receipt of a letter from Keyes who asks him to extend protected status to the American bald eagle.
This is part of an article by Charles Keyes comparing alliteration and rime in the English and German languages from the Proceedings of the American Philology Association, v. xxxiii, 1902