O`Beirne, Brenda R.
Allyn and Bacon, 1997
Buku Teks
Rommen, Heinrich A.
Liberty Fund Indianapolis, 1998
Buku Teks
Levinson, Paul, author
Routledge, 1977
Buku Teks
Linton, Alan H., author
John Wiley & Sons, 1982
Buku Teks
Mannion, A. M.
Longman Scientific & Technical, 1991
Buku Teks