Webster's guide to business correspondence
Merriam-Webster, Inc., 1988
 Buku Teks SO
Merriam-Webster's guide to business correspondence : a merriam-webster
Merriam-Webster's, Incorporated, 1993
 Buku Referensi
Guffey, Mary Ellen
Essentials of business communication
PWS-Kent, 1988
 Buku Teks
Guffey, Mary Ellen
Essentials of business communication
Cengage Learning Asia, 2008
 Buku Teks SO
Guffey, Mary Ellen
Essentials of business communication
PWS Publishing, 1991
 Buku Teks SO