In his book, Off the Books: The Underground Economy of the Urban Poor, sociologist Sudhir Venkatesh explores the thriving informal economy that exists off the books in many inner cities. Rather than being strictly limited to illegal activities, this cash- and barter-driven economy provides a wide variety of goods and services to those who utilize it, but can also exact a toll on those who partake.
See "Urban Poor Cope with Help from Informal Economy", Jim Zarroli, All Things Considered, March 15, 2007