Graham, Robert
St. Martin's Press, 1979
Buku Teks SO
Brown, Arthur W.
Twayne Publishers, 1964
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Artikel Jurnal
edited by Geatoid O`Tuathail and Simon Dalby
Routledge, 1998
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Flint, Colin
Routledge, Taylor & Francis Group, 2017
Buku Teks SO