edited by Stephen White, Judy Batt and Paul G. Lewiss
Macmillan, 1993
Buku Teks SO
Black C.E.
Rutgers Univ. Press, 1954, 1954
Buku Teks
Skilling, Gordon H.
Thomas Y. Crowell, 1966
Buku Teks SO
Robbins, Peter T.
Springer, 2011
eBooks
Seton-Watson, Hugh
Methuen , 1950
Buku Teks SO