Gultom, Samuel
Greenwood Press, 1987
Buku Referensi
VanSpanckeren, Kathryn
Lembaga Penerangan Amerika Serikat, 2001
Buku Teks
edited by Drinkwater, John; revised by Horace Shipp
Newnes, 1957
Buku Teks SO
Needleman, Morris H.
Barnes & Noble, 1939
Buku Teks
Otis, William Bradley
Barnes & Noble, 1939
Buku Teks