Verhagen, Arie, author
Oxford University Press, 2007
Buku Teks
Adler, Patricia A., author
Wadsworth, 2000
Buku Teks
Encyclopaedia Britannica Inc., 2002
Buku Referensi
Encyclopaedia Britannica Inc., 1998
Buku Referensi
Funk & Wagnalls, 2000,
Buku Referensi