Knapp, Brian J.
Longman, 1992
Buku Teks
Jackson, Andrew R.W.
Longman, 1996
Buku Teks
edited by Moryteza Honari and Thomas Boleyn
Routledge, 1999
Buku Teks SO
Meyer, Evelyn E.
World Health Organization, 1975
Buku Teks SO
by Institution Syposium Smithsonian
Voice of American Forum Lectures, 1968
Buku Teks