Twelvetrees, Alan
Macmillan, 1982
Buku Teks
Henderson, Paul, author
Routledge, 1996
Buku Teks
Clarke, Stephen, author
Routledge, Taylor and Francis Group, 2000
Buku Teks
McIvor, Gill
Avebury, 1992
Buku Teks
Brownlee, Ian, author
Longman, 1998
Buku Teks