Primary:

• 1887. "The arithmetizing of mathematics" in Ewald, William B., ed., 1996. From Kant to Hilbert: A Source Book in the Foundations of Mathematics, 2 vols. Oxford Uni. Press: 965–71.
• 1921. "Felix Klein gesammelte mathematische Abhandlungen" R. Fricke and A. Ostrowski (eds.) Berlin, Springer. 3 volumes. (online copy at GDZ)
• 1890. "Nicht-Euklidische Geometrie"

Secondary

• Mumford, David, Carol Series, and David Wright Indra's Pearls: The Vision of Felix Klein. Cambridge Univ. Press. 2002.
• Tobies, Renate (with Fritz König) Felix Klein. Teubner Verlag, Leipzig 1981.