Strong, C.F.
Sidgwick & Jackson Limited, 1960
Buku Teks
Hague, Rod.
Palgrave Macmillan , 2013
Buku Teks SO
Roucek, Joseph S.
Thomas Y. Crowell Company, 1950
Buku Teks
Field, G. Lowell
Cornell University Press, 1967
Buku Teks
Lentera Abadi, 2010
Buku Referensi