English, Jack
McGraw-Hill, 1992
Buku Teks
Blueprint, 1992
Buku Referensi
Campbell, Don G., 1946-, author
MAcDonald and Evans , 1978
Buku Teks
Joan, Dean, author
Routledge , 2001
Buku Teks
Westring, Gosta B.
N.Y. United Nations Institute for traning and research , 1974
Buku Teks