Hartley, Michael G.
McGraw-Hill, 1978
Buku Teks
Giarratano, Joseph C., author
Howard W. Sams, 1982
Buku Teks
United Nations , 1971
Buku Teks
Amoroso, Edward G., author
Prentice-Hall, 1994
Buku Teks
Chapelle, Carol A, author
Cambridge University Press, 2006
Buku Teks