Routledge, 1991
Buku Teks
Interscience Publishers, 1956
Buku Teks
Brace, Richard M., author
Holt, Rinehart and Winston, 1961
Buku Referensi
Hay, Denys
London Methuen & Co 1953
Buku Teks
Weber, Eugen, author
Boston: D.C.Heath and Company, (1959).
Buku Teks