why is this considered a "great work" of literature

I'm not sure who Allen Barra is but I'd have to disagree, pretty strongly. I'd also end up saying something rude to him. I always defined a classic by how long it stayed in my head. I read Mockingbird in grade eight; let's just say that was a long time ago. I still think about this book at least a few times a week and not because it is on GradeSaver. I often think about Atticus and what he might do in a given situation. Now I'm certainly no Atticus Finch but at least I know he'd do the right thing. To Kill a Mockingbird has to be one of those books that, when mentioned, people of all backgrounds will wax nostalgic about. They remember when and where they were when they read it as a fond smile crosses their face. I'm not exactly sure the academic criteria that makes a book a classic but as far as I'm concerned, and countless others, Mockingbird most certainly is.