Allen, R.G.D., author
Macmillan, 1980
Buku Teks
by United Nation
United Nations, 1991
Buku Teks
Fraser, D.A.S., author
John Wiley & Sons, 1967
Buku Teks
Rickmers, Albert D., author
McGraw-Hill, 1967
Buku Teks
Walpole, Ronald E., author
Macmillan, 1968
Buku Teks