Ayer, Alfred Jules, 1910-1989
Freeman, Coope, 1968
Buku Teks SO
White, James R.H.
McGraw-Hill, 1975
Buku Teks
Carr, Emily
Clarke Irwin, 1946
Buku Teks SO
Brown, Bethel
John Wiley & Sons, 1958
Buku Teks SO
Robert Maynard Hutchins, editor in chief
Encyclopaedia Britannica, Inc., 1952
Buku Referensi