Schulz, Ernest B
[publisher not identified], 1956
Buku Teks
Arblaster, Anthony
Open University Press, 1987
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Boix, Carles
Cambridge University Press, 2003
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Padover, Saul K. (Saul Kussiel)
Praeger, 1967
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Nagle, John D.
Sage Publications, 1999
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