Coles, Robert.
Random House, 1997
Buku Teks
Coles, Robert
The Atlantic Monthly Press, 1986
Buku Teks
Spaemann, Robert
Routledge, 1991
Buku Teks
Long, Roberrt H.
Robert Morris Associates , 1990
Buku Teks
Cooper, Robert K., author
Orion Business Books, 1997
Buku Teks