Here's a great book to check out, particularly if you're new to documentary film and want to learn more about its history and sub-genres. Film critic Patricia Aufderheide's Documentary Film: A Very Short History lives up to its title while providing a useful overview.
To help define documentary, Aufderheide writes: "A simple answer might be: a movie about real life. And that is precisely the problem: documentaries are about life; they are not real life. They are not even windows onto real life. They are portraits of real life, using real life as their raw material, constructed by artists and technicians who make myriad decisions about what story to tell to whom, and for what purpose."
By briefly cataloging the wide range of films that all fall within the documentary genre, she helps refute some of the the criticisms typically issued by right-wing pundits at contemporary documentary releases. Filmmakers like Michael Moore are often under fire for a perceived lack of objectivity or neutrality, but as Aufderheide reminds us, "Broadcast journalist Edward R. Murrow once said, 'Anyone who believes that every individual film must represent a 'balanced' picture knows nothing about either balance or pictures.'"