Holschuh, Louis W.
New York St. Martin's Press 1991,
Buku Teks
by L.G. Alexander ... [and others]
Longman, 1975
Buku Teks SO
Harahap, P.R.
Jakarta Praninta Aksara [date of publication not identified],
Buku Teks
Klatt, Edmund
Berlin Langenscheidt KG. Verlag buchhandlung 1961
Buku Referensi
London; Cassell LTD., 1978
Buku Referensi