Blueprint, 1992
Buku Referensi
Campbell, Don G., 1946-
MAcDonald and Evans , 1978
Buku Teks
Joan, Dean
Routledge , 2001
Buku Teks
Westring, Gosta B.
N.Y. United Nations Institute for traning and research , 1974
Buku Teks
Guth, Hans P.
Wadsworth, 1969
Buku Teks