Shepherd, William G.
Richard D. Irwin, 1979
Buku Teks
Backman, Jules
Ronald Press, 1953
Buku Teks
Shepherd, Geoffrey S.
Iowa State University Press, 1968
Buku Teks
edited by James C. Miller III and Bruce Yandle
American Enterprise Institute for Public Policy Research, 1979
Buku Teks SO
Warntz, William
University of Pennsylvania Press , 1959
Buku Teks