edited by Holly Koelling
American Library association, 2007
eBooks
R.R. Bowker, 1991
Buku Teks
edited by John P. Holdren, Eric Donald Hirsch
Delta Pub., 1996
Buku Teks
Reader's Digest Association , 1962
Buku Teks
Robert M. Hutchins, Mortimer J. Adler, editors in chief
Encyclopaedia Britannica, Inc., 1963
Buku Referensi