Turnbull, Harley, author
Glorier International, 2001
Buku Referensi
Michael R. Czinkota, Ilkka A. Ronkainen, Michael H. Moffet, Eugene O. Moynihan
Harcourt College publishers, 2001
Buku Teks
Globerman, Steven
Prentince-Hall, 1986
Buku Teks
Johnson, Debra, author
Routledge, Taylor & Francis group, 2003
Buku Teks
Moran, Robert T., author
Gulf, 1991
Buku Teks