Pye, Keith
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Buku Teks
Renwick, Alan.
Stanley Thornes Publishing, 1993
Buku Teks
Evans, Graham, author
Harvester Wheatsheaf, 1990
Buku Referensi
Grayson-Smith, Hugh, author
Prentice-Hall, 1967
Buku Teks
Buchanan, James M., author
Liberty Fund, 2000
Buku Referensi