Euromonitor, 1989
Buku Teks
Todaro, Michael P., author
Longman, 1992
Buku Teks
Handelman, Howard, author
Pearson/Prentice Hall, 2006
Buku Teks
Pinkney, Robert, author
Open University Press, 1993
Buku Teks
Gould, W.T.S., author
John Wiley, Longman Scientific, 1993
Buku Teks