Johnson, Elmer D., author
The Scarecrow Press, 1970
Buku Teks
editor in chief, Mortimer J. Adler
Encyclopedia Britanica, 1990
Buku Referensi
McKay, John P., author
Houghton Mifflin , 1983
Buku Teks
Adams, Laurie Schneider, author
Brown & Benchmark , 1997
Buku Referensi
editor in cheief, Mortimer J. Adler
Encyclopedia Britanica, 1996
Buku Referensi