Hemphill, Charles F., author
Prentice-Hall, 1980
Buku Teks
Lippincott , 2004
Buku Teks
edited by Coleen K. Withgott, Austin G. Anderson
Merriam-Webster, 1981
Buku Referensi
Bolander, Donald O., author
Career Publishing, 1992
Buku Referensi
M. Ikhsan Modjo, author
United Nations Development Programme (UNDP), 2009
Buku Teks