The first book I ever read by Howard Zinn was SNCC: The New Abolitionists. I've read a lot of his writing since then, and I think it's his most powerful book.
Howard Zinn wrote an essay The Optimism of Uncertainty. He argued that history should give us hope, not because it guaranteed that the powerless would win (it really doesn't), but because it showed extraordinary, unpredictable change is possible. The Civil Rights Movement, particularly SNCC, is an example of the unpredictability of hope. On the 1st of February 1960, Ezell A. Blair Jr., David Richmond, Joseph McNeil, and Franklin McCain, sat down at the counter of their local Woolworth's and refused to be served. Nobody could have predicted what would grow out of that action.
There have been so many attempts to hide the history of collective resistance, including the reduction of the the freedom movement SNCC was part of to someone sitting down on a bus and someone else giving a great speech. Howard Zinn wrote history like it mattered, because he wanted to cultivate the hope that history brings.