Easton, David, author
Prentice-Hall, 1965
Buku Teks
Jacob, Herbert, 1933-, author
McGraw-Hill, 1975
Buku Teks
Strong, C.F., author
Sidgwick & Jackson Limited, 1960
Buku Teks
Easton, David, author
New York, N. Y. Wiley 1967,
Buku Teks
Maxey, Chester C., author
Macmillan, 1961
Buku Teks