New York : Holt, Rinehart and Winston, 1995,
Buku Teks
Holschuh, Louis W.
New York St. Martin's Press 1991,
Buku Teks
Longman Group UK Limited , 1975
Buku Teks
Harahap, P.R.
Jakarta Praninta Aksara [date of publication not identified],
Buku Teks
Klatt, Edmund
Berlin Langenscheidt KG. Verlag buchhandlung 1961
Buku Referensi