Howard Zinn, an influential leftist historian whose books are widely assigned on college campuses, died Wednesday at the age of 87. Zinn was politically active throughout his career, which included teaching positions at Boston University and other institutions. At BU, he was both a critic and target of John Silber during his presidency there. Of Zinn's books, the most influential is A People's History of the United States, which describes itself as "American history from the bottom up, throwing out the official version of history taught in schools -- with its emphasis on great men in high places -- to focus on the street, the home, and the, workplace." Zinn's Web site features numerous essays, interviews and a bibliography.