Classic literature is the recordings of the human heart and spirit, and, thus, has universal appeal. Of paramount importance to any culture, literature is immortalized truth that is not tampered with; it is the veritable outpourings of men's souls, a truth that is renewed over and over with the recordings of other poets and writers....semper veritas. To underscore this idea, French philosopher Denis Diderot wrote, "The truest history is full of falsehoods, and your romance if full of truths."
Classic literature provides not just enjoyment, but also understanding, expanding and refining the mind of the reader, thus illuminating people's sense of life. Indeed, it is one of life's greatest instructors and comforters. Interpretive literature, critics agree, has as an objective both pleasure and understanding. Ralph Waldo Emerson observed, "For only that book can we read which relates to me something that is already in my mind." Literature, thus, also connects the reader with the community of man, who, like the reader, has struggled, wept, lost, and known triumphs in life.
First, I think it is important to note that we cannot say that every single book ever published can be considered literature. In the vast vast world of book publication, I think the idea of "universal appeal" is what differentiates literature from everything else.
Universal appeal is the idea that something about the book touches the human spirit, evokes human emotion, or makes a personal connection through human relationships. And this happens across national boundaries, across time barriers, across gender differences, and across historical backgrounds. It has the ability to evoke common human responses, universally. And the beauty of literature is that this can be done through more than just the telling of a good story. Universal appeal can be achieved through language, themes, style, and other literary devices.
To answer your question concisely, I believe that in order for a book to be considered "literature," it musthave universal appeal. It must be able to touch the human spirit across the test of time, geography, and experience.