Shultz, Richard D.
John Wiley & Sons, 1988
Buku Teks
Nixon, Richard, author
Simon & Schuster Building, 1990
Buku Teks
Shultz, Richard D.
Harper & Row Pub., 1985
Buku Teks
Varga, Richard S.
Prentice-Hall, 1962
Buku Teks
Roberts, Richard A., 1935-, author
Massachusetts Addison-Wesley, 1992
Buku Teks