Ripley, William Z.
Little, Brown, and Company, 1927
Buku Teks
Crum, William Leonard
University of California Press, 1953
Buku Teks
Robbins, Sydney M.
Houghton Miffiln, 1954
Buku Teks
Dewing, Arthur Stone
Ronald Press Co, 1953
Buku Teks
Hoagland, Henry E.
McGraw-Hill Book Company, Inc., 1947
Buku Teks