Hall, Kermit L.
Oxford University Press, 1991
Buku Teks
Hoffman, Herbert H.
Scarecrow Press, 1985
Buku Referensi
Crawford, William R.
Frederick A. Praeger, 1961
Buku Teks SO
edited by Howard J. Wiarda, Harvey F. Kline
Westview Press, 2007
Buku Teks SO
Wynia, Gary W.
Cambridge University Press , 1984
Buku Teks SO