Abel, Darrel
Barron's Educational Series, 1963
Buku Referensi
Duke University Press,
Majalah, Jurnal, Buletin
compiled by Ronald Gottesman ... [and others]
W.W. Norton, 1989
Buku Teks SO
Soenarjati Djajanegara
2005
Artikel Jurnal
Gultom, Samuel
Greenwood Press, 1987
Buku Referensi