Letta, Corrado G.M., author
Program Pascasarjana Universitas Indonesia, 2006
Artikel Jurnal
Dupee, F.W., author
Doubleday, 1956
Buku Teks
Swan, Michael, author
Longmans, 1950
Buku Teks
Swan, Michael, author
Longmans Green, 1969
Buku Teks
Becker, George Joseph, author
Frederick Ungar Publishing, 1983
Buku Teks