Scott-Kilvert, Ian
John Webster
Longmans, Green, 1970
Buku Teks
Harrower, Elizabeth, 1928-
The watch tower
Angus & Robertson, 1966
Buku Teks
Merriam-Webster's guide to business correspondence : a merriam-webster
Merriam-Webster's, Incorporated, 1993
Buku Referensi
Merriam-Webster's collegiate encyclopedia
Merriam-Webster, 2000
Buku Referensi
Merriam-Webster's desk dictionary
Merriam-Webster, 1995
Buku Referensi