Rogger, Hans, author
Longman, 1992
Buku Teks
Kowalski, Ronald
Routledge, 1997
Buku Teks
Wade, Rex A, author
Cambridge University Press, 2017
Buku Teks
Moorehead, Alan, author
Harper and Brother, 1958
Buku Teks
McVey, Ruth Thomas, author
Cornell university, 1962, 1957.
Buku Teks