Overview: USDA’s Food and Nutrition Service (FNS) provides program overviews with extensive links:
USDA’s Economic Research Service (ERS) has an Overview, an annual report on the Food Assistance Landscape, and periodic summary reports of background and trends for entire program areas, including WIC and NSLP.
Authorization: SNAP is periodically reauthorized as part of the Farm Bill (most recently the Agriculture Act of 2014). WIC and the child nutrition programs are part of child nutrition reauthorization (most recently the Healthy Hunger-Free Kids Act of 2010).
Regulations: The authoritative source is the Code of Federal Regulations, but a good way to find the right link for each program area is from the FNS website.