Longman Group UK Limited , 1975
Buku Teks SO
New York : Holt, Rinehart and Winston, 1995,
Buku Teks
Robert, Collin
Paris : Harper Collins Publisher, 2000
Buku Referensi
Harahap, P.R.
Jakarta Praninta Aksara [date of publication not identified],
Buku Teks
Klatt, Edmund
Berlin Langenscheidt KG. Verlag buchhandlung 1961
Buku Referensi