Gultom, Samuel
Greenwood Press, 1987
Buku Referensi
VanSpanckeren, Kathryn
Lembaga Penerangan Amerika Serikat, 2001
Buku Teks
Needleman, Morris H.
1950
Buku Teks
Otis, William Bradley
Barnes & Noble, 1939
Buku Teks
Abel, Darrel
Barron's Educational Series, 1963
Buku Referensi