Westoby, Adam
Free Press, 1989
Buku Teks
Houn, Franklin W.
Prenrice-Hall , 1973
Buku Teks
Plamenatz, John, author
Longmans, Green, 1954
Buku Teks
Webb, Sidney, author
Longman, 1947
Buku Teks
edited by Silvio Pons ... [et al.]
Cambridge University Press, 2017
Buku Teks