Quite a bit of the people's history of the united states is primary sources.

Zinn decided to bias the viewpoint towards oppressed people, the same way many textbooks bias it in favor of the 'winning' party. While I felt like I got a pretty decent education in high school history, Zinn didn't invalidate what I learned but showed quite clearly how these things affected others.

When I was in school it would have been highly controversial to compare chattel slavery or the conquering of the New World to the holocaust but the reality is the severity is similar, just on a different timeframe.