Crain, William C, author
Prentice-Hall, 2005
Buku Teks
Hess, Peter
Harcourt Brace & Company, 1997
Buku Teks
Muhammad Ismail
Kementerian Dalam Negeri Ri,
Artikel Jurnal
Oxford University Press, 2003
Buku Teks
Boehm, E.A., author
Longman Chesire, 1979
Buku Teks