Numrich, Carol, author
Longman: Addison-Wesley Pub., 1995
Buku Referensi
Allen, Edward David, author
Harcourt Brace Jovanovich, [Date of publication not identified]
Buku Teks
Katzner, Kenneth, author
Routledge, 1995
Buku Referensi
edited by Diane Brentari
Cambridge University Press, 2010
eBooks
edited by Diane Brentari
Cambridge University Press, 2010
Buku Teks