Karl Menninger: Number words and number symbols: a cultural history of numbers; translated by Paul Broneer from the revised German edition. Cambridge, Mass.: M.I.T. Press, 1969 (also available in paperback: New York: Dover, 1992 ISBN0-486-27096-3)
Levi Leonard Conant: The Number Concept: Its Origin and Development; New York, New York: Macmillan & Co, 1931. Project Gutenberg EBook