Lesikar, Raymond V. (Raymond Vincent), author
McGraw-Hill/Irwin , 2002
Buku Teks
Wilson, Kevin, author
American Management Association, 2010
eBooks
Merriam-Webster, Inc., 1988
Buku Teks
Merriam-Webster's, Incorporated, 1993
Buku Referensi
Himstreet, William C.
Kent Publishing Company, 1984
Buku Teks