The Statistical Abstract of the United States is a great compilation of national statistics from many sources. It can also be used as an index, or finding tool, to find the source of the statistics... and much more data.
The Green Book is the standard reference for social policy and federal entitlement programs. Statistics are often included.
The U.S. Census Bureau provides local, state and national statistics relating to poverty, educational attainment, overcrowded housing, single parent families, grandparents raising grandchildren, etc. Much of the data can be "cross-tabulated" by another variable such as race/ethnicity, gender, immigrant status, language or age. American FactFinder is the main entry point. It's complicated; we've created a guide to finding poverty-related census data.
See Data and Statistics for a much more comprehensive list of social welfare related statistical sources.