Townsley, W.A.
University of Queensland Press, 1976
Buku Teks
Lancaster, Lane W.
Van Nostrand , 1952
Buku Teks
Merritt, Allen S
Fitzhenry & Whiteside, 1961
Buku Teks
World Bank, 2002
Buku Teks
Hoan, Daniel W.
Harcourt, Brace and Company, 1936
Buku Teks