Wiarda, Howard J., author
Thomson/Wadsworth, 2004
Buku Teks
Westview Press , 1985
Buku Teks
Cammack, Paul Anthony, author
Leicester University Press, 1997
Buku Teks
Hettne, Bjorn 1939-
Longman Scientific & Technical , 1995
Buku Teks
Deegan, Heather, author
Routledge, 1996
Buku Teks