Readers Digest, 1985
Buku Referensi
White, Roderick, author
McGraw-Hill, 1980
Buku Teks
White, Roderick, author
McGraw-Hill, 1993
Buku Teks
Allen, William Richard, 1924-, author
American Management Association, 1993
eBooks
The Reader's Digest Association, 1988
Buku Referensi