Williams, L.A.
London Longmans, Green and Co. Ltd. 1967,
Buku Teks
Williams, L.A. (Leonard Aimslie), 1909-
American Elsevier Publishing Company, INC, 1968
Buku Teks
Wilson, R. M. S., author
Heinemann, 1973
Buku Teks
Haas, Robert W
Kent, 1982
Buku Teks
Douglas M. Considine, editor-in-chief.
McGraw-Hill, 1993
Buku Referensi