Cornog, Mary Wood
Springfield:Massachusetts Merriam-Webster, 1994,
Buku Teks
New York : Holt, Rinehart and Winston, 1995,
Buku Teks
Nababan, Melva Ratna Rulida
Fakultas Ilmu Pengetahuan dan Budaya Universitas Indonesia, 1983
UI - Skripsi Membership
Longman Group UK Limited , 1975
Buku Teks
Harahap, P.R.
Jakarta Praninta Aksara [date of publication not identified],
Buku Teks