Wilson, R. M. S., author
Heinemann, 1973
Buku Teks
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
edited by Charles P. Bonini, Robert K. Jaedicke and Harvey M. Wagner
McGraw-Hill, 1964
Buku Teks
Keller, kotler, author
Pearson Education, Inc, 2006
Buku Teks