Wilson, R. M. S., author
Heinemann, 1973
Buku Teks
Williams, Leonard Aimslie
London Longmans, Green and Co. Ltd. 1967,
Buku Teks
Williams, Leonard Aimslie
American Elsevier Publishing Company, INC, 1968
Buku Teks
edited by Charles P. Bonini, Robert K. Jaedicke and Harvey M. Wagner
McGraw-Hill, 1964
Buku Teks
Kotler, Philip, author
Prentice-Hall, 2000
Buku Teks