Artikel Jurnal
edited by K.A. Joysey and A.E. Friday
Academic Press, 1981
Buku Teks
Mohammad Iqbal, author
Kitab Publishing House, 1974
Buku Teks
Christopher Ehret and Merrick Posnansky edited by
University of California Press, 1982
Buku Teks
World Bank, 2005
Buku Teks