Shacford, R.H., author
Public Affairs Press, 1952
Buku Teks
Lamont, Corliss, author
Philosophical Library, 1952
Buku Teks
Hough, Jerry F.
The Brookings Institution, 1980
Buku Teks
Frederick A, Praeger; George Allen and Unwin Ltd., 1964
Buku Teks
McCauley, Martin
Longman, 1993
Buku Teks