Morton, Joyce
Prentice-Hall, 1998
Buku Teks
edited by Coleen K. Withgott, Austin G. Anderson
Merriam-Webster, 1981
Buku Referensi
De Vries, Mary Ann
Prentice-Hall, 1991
Buku Referensi
prepared under the editorial supervision of In Plain English, Inc.
Macmillan, 1996
Buku Referensi
Meyer, Carl H.
John Wiley & Sons, 1982
Buku Teks SO