edited by Susan Hornby, Zoe Clarke
Facet, 2004
Buku Teks SO
edited by Gerald W. Whitted
Pan American World Airways, 1971
Buku Referensi
Barnes, Harry Elmer
Prentice-Hall of India, 1966
Buku Teks
edited by John Lyons
Penguin Books, 1973
Buku Teks SO
edited by Gerald W. Whitted
Pan American World Airways, 1963
Buku Referensi