The Reader's Digest Association, 1988
Buku Referensi
Turock, Betty J., author
Neal-Schuman Publishers, 1992
Buku Teks
Hahn, Fred E., 1925-, author
Grasindo , 1999
Buku Teks
Aitchison, Jim, author
Prentice-Hall, 1999
Buku Teks
Aitchison, Jim, author
Prentice-Hall, 1999
Buku Teks