Brewster, Lawrence G., author
Thomson Wadsworth, 2004
Buku Teks
Culver, John H.
Macmillan Publishing Company,
Buku Teks
Combs, James E., author
Macmillan, 1984
Buku Teks
Lonely Planet Publication, 2003
Buku Teks
Bell, Charles G.
The Dorsey Press, 1980
Buku Teks