Artikel Jurnal
Baldwin, Ralph B.
McGraw-Hill, c1965
Buku Teks
edited by Horace Belshaw
University of California Press, 1947
Buku Teks SO
Thakur, Ramesh
London: Westview Press, 1986,
Buku Referensi
Boyce, Peter John
University of Queensland Press, 1977
Buku Teks