Graver, B.D.
Oxford University Press, 1990
Buku Teks
Hewings, Martin, author
Cambridge University Press, 2005
Buku Teks
Kruse, Benedict, author
McGraw-Hill, 1976
Buku Teks
Thomas Cable
Prentice-Hall, 1993
Buku Teks
Alexander, L.G., author
Longman, 1967
Buku Teks