Williamson, Harold F.
Prentice-Hall, 1951
Buku Teks SO
Clarendon Press, 1994
Buku Teks
Fogel, Robert William
Johns Hopkins Press, 1964
Buku Teks
Dam, Kenneth W.
The Brooking Institution, 2006
Buku Teks
Kohona, Palitha T.B.
Martinus Nijhoff, 1985
Buku Teks