Easton, David, author
Knopf, 1971
Buku Teks
Easton, David, author
John Wiley & Sons, 1965
Buku Teks
Clinton, Richard L., author
Lexington Books, 1972
Buku Teks
Baglione, Lisa A., author
Thomson Wadsworth, 2007
Buku Teks
University of London , 1987
Buku Teks