Social Work Almanac, 2nd ed (book) by 
Statistical Handbook on the Social Safety Net (book) by 
Statistical Handbook on the World's Children by Subject encyclopedias provide an overview of your topic, often including history, current state, policy issues & relevant theories, while specialized dictionaries provide definitions of terms or phrases unique to the field.
Comprehensive Handbook of Social Work and Social Welfare, the Profession of Social Work (book) by 
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Encyclopedia of Measurement and Statistics (book) by 
Encyclopedia of Social Welfare History in North America (book & e-book) by 
Gay Histories and Cultures: An encyclopedia (book) by 
A Dictionary of Social Work and Social Care, 2nd ed (E-book) by Encyclopedia of Social Welfare History in North America (book & e-book) by 
Encyclopedia of Social Work by 
Social Policy: Essential Primary Sources (E-book) by