Best known for his classic, A People's History of the United States, Zinn changed the way many think about America.  The book, written from an unapologetic liberal stance, decided not to glorigy America and its founders, but noted the struggles of real Americans in making change.  Zinn himself would admit that the book has a biased slant — and it did.  But it was also accurate, and celebrated the capabilities of everyday people to make tremendous change.  A must read.