This was a very important book in American history because it led to many of the social programs that were put in place by Pres. Johnson in the 1960s. In this book, Harrington argued that there were many poor people in America but that those poor people were being overlooked because of the general prosperity that most Americans were enjoying at the time.
In the early 1960s, America was still in its "golden age" that had started in the 1950s. The idea was that everyone was doing well and that the country was booming. Harrington argued that there was a large group of Americans (40 or 50 million) who were poor. He said that they were not starving or about to die, but that they did not have all of the "basic necessities of life."
Harrington's book was important because it put people like this more in the public eye and made American policy makers more aware of their existence.