Use secondary sources to:
Below you will find Reference Books and Journal Article Databases.
You may also want to check out the Social Welfare History Group website. A division of the American Historical Association (also closely affiliated with NASW and CSWE), the SWHG "encourage[s] the teaching of history and the value of doing historical research on social work and social welfare through presentations at conferences, publication of bibliographies and articles and dissertations."
Some recommendations for specific titles. You can also use a reference database like SAGE Knowledge to search across multiple titles at once. Or identify other subject-specific encyclopedias and handbooks in the library catalog.