I suppose Watchmen is the example. I disagree with calling that a "graphic novel." Yes, it was conceived as a finite story. However, it was also conceived as individual issues of a comic book. Ergo, the whole thing is a collection. To me, the word "novel" implies not only a complete story, but also a standalone product.
The Christian Bible. One book, or many books?
"The trouble with being a ghost writer or artist is that you must remain anonymous without credit.
If one wants the credit, one has to cease being a ghost and become a leader or innovator." — Bob Kane