Reference sources are invaluable for defining key concepts, providing brief overviews of a subject, and identifying major theories and research developments. They can provide you with the background you need to select and develop a topic of research, or help you explore internship or career opportunities.
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Encyclopedia of Social Theory (book) by 
Encyclopedia of Sociology, 2nd ed (book) by 
The SAGE Dictionary of Sociology (book) by 
The SAGE Encyclopedia of Social Science Research Methods, 3 vols (book) by 
Information Sources in the Social Sciences (book) by