I know this, but in the constant barrage of content and shortening attention spans we have today, movies are keeping some big stories from the past alive, and are becoming part of the universe begun by the books.

I assumed OP was talking about books that had a long lasting impression that were commercially successful today. Charles Dickens and Mark Twain were big for their time, but I wouldn't expect an adaptation of Great Expectations to generate billions in revenue the way Harry Potter did.