Deb Ellis and Dennis Mueller's long-overdue profile of Howard Zinn, now 81, is narrated and funded in part by Matt Damon, whose GOOD WILL HUNTING (1997) paid homage to the radical historian and activist. The subtitle, taken from Zinn's 1994 memoir, is his response to students who question his objectivity. "Certain events are already moving in certain deadly directions," he writes, "and to be neutral means to accept that." In Zinn's worldview, neutrality equals collaboration, and his life of political engagement embodies his belief that it's incumbent upon thinking individuals to choose a side and intercede in history. Raised poor on New York City's Lower East Side, Z...