To locate books, try GW Library Search - Advanced Search using the term human services, or related terms, e.g., social care, social work, etc. Add other search terms in the additional search boxes that relate to your topic. The same would apply for finding materials on social justice issues. Specific examples might include:
human services AND administration
human services AND policy
social care AND elders
social work AND research
social welfare AND homeless
human services AND disability
social justice AND medical practice
social justice AND native americans
social justice AND discrimination
social mobility AND african americans
inequality AND women AND employment
discrimination AND latinos AND housing
Books on human services may be found mainly in these call number ranges:
HV40-69 Social service. Social work. Charity. Organization and practice
HV85-525 By region or country
HV544.5 International social work
Works on social justice vary in location depending on the specific topic (e.g., social justice and housing, social justice and race, social justice and education, etc.), but a number of the more general titles may be found in the HM 661-696 (social control) area.
Books in the above ranges are located on the 5th floor of Gelman. See the "Gelman Stacks Map" to the left.