Livermore, H.V., author
Cambridge University Press, 1969
Buku Teks
Office of the Secretary of State for Information and Tourism, [date of publication not identified]
Buku Teks
Disney, A.R, author
Cambridge University Press, 2009
Buku Teks
Birmingham, David, author
Cambridge University Press, 1993
Buku Teks
Torga, Miguel, author
Rua Cintura do Porto, 2003
Buku Referensi