United Nations University, 1993
Buku Teks
Marcus, Edward, author
Pitman, 1965
Buku Teks
White, Robert W., author
Prentice-Hall, 1996
Buku Teks
Dunning, John H.
George Allen and Unwin , 1974
Buku Teks
Gipson, Carolyn R., author
McGraw-Hill, 1994
Buku Referensi