Treue, Wolfgang, author
Inter Nationes, 1969
Buku Teks
Cook, Chris, author
Longman, 1991
Buku Referensi
Cook, Chris, author
Routledge, 2005
Buku Teks
Keir, David Lindsay, author
Adam and Charles Black, 1961
Buku Teks
Smith, Bonnie G., author
D.C. Heath, 1989
Buku Teks