Elcock, Karen, author
Churchill Livingstone, 2011
Buku Teks
Vallerand, April Hazard, author
F.A.Davis Company, 2014
Buku Teks
Wilson, Billie Ann, author
Prentice-Hall, 2003
Buku Referensi
Richards, Ann, author
Elsevier, 208
Buku Teks
Crow, Lester D
Macmillan, 1964
Buku Teks