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