edited by Muriel Albers Bittel, Lester Robert Bittel and Jackson Eugene Ramsey
Grolier International, 1995
Buku Referensi
Madura, Jeff
Thomson South-Western, 2003
Buku Teks
Allen, David
Crest Publishing House, 2002
Buku Teks
Madura, Jeff
South-Western College, 1998
Buku Teks
Rasmussen, A. Eric
Co-operative College of Canada, 1975
Buku Teks