Ellis, David
Progress and problems in information retrieval
Library Association Publishing, 1996
 Buku Teks
Hill, Norman
International organization
Harper & Brothers, 1952
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Colm, Gerhard
The economy of the American people : progress, problems, prospects
National Planning Association , 1961
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Problems and progress in medical care: essays on current research
edited by Gordon Mclachlan
Nuffield Provincial Hospital Trust, 1972
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Problems and progress in medical care: essays on current research
edited by Gordon Mclachlan
Oxford University Press, 1970
 Buku Teks