Longman Group UK Limited , 1975
Buku Teks
Harahap, P.R.
Jakarta Praninta Aksara [date of publication not identified],
Buku Teks
Krause, Wayne B
Jakarta: Elex Media Komputerindo, 2002
Buku Teks
Klatt, Edmund
Berlin Langenscheidt KG. Verlag buchhandlung 1961
Buku Referensi
London, London Longmans
Buku Referensi