A few ideas:
The biography of the writer.
The time period when a book was written.
How the book was published:
serialized, like Twain's work...Harriet Beecher Stowe's Uncle Tom's Cabin for example
self-published v traditional-published v ebook
Was the book translated? This can certainly have an effect.
More than one version, printing, edition, publishers, etc.
Has the book been censored?
All these characteristics contribute to the understanding of a book.
Hope this helps!