Robertson, Christopher R.
Edward Arnold, 1993
Buku Teks
Bethune, James D.
Prentice-Hall, 1980
Buku Teks
Morris, Noel Malcolm, 1929-
Longman, 1980
Buku Teks
Hughes, Edward
Pearson Education, 2002
Buku Teks
Green, D.C.
Longman Scientific & Tecnical, 1992
Buku Teks