Phillips, Richard, 1965-, author
Sage, 2012
Buku Teks
Senior, Michael, author
Longman, 1979
Buku Teks
Artikel Jurnal
Von Mises, Ludwig, 1881-1973, author
Yale University Press, 1963
Buku Klasik
De Blij, Harm J., author
John Wiley & Sons, 1977
Buku Teks