Something in the sentence "I'm writing a book" sounds awry. Wrong, even. "I'm writing a story" would be much better. Best would be "I'm writing a text", wouldn't it? Where do these names start, where do they end?
A Text, of that much I am certain, is written stuff. (In this context, that is. In culture studies, just about anything can be labeled a text, films and video games included.) The story is already harder to define. As far as I am concerned, it denotes the content of a text, given we're talking about fiction. This content, however, isn't in any fixed shape. It's more of an abstract thought than a physical entity. The text, of course, is a fixed combination of words, but a story can take on many different forms. It can be adapted, extended, changed and redone and still be recognizable. The story could therefore be called the essence of the text.
A writer usually sells both when she signs with a publisher. She sells the text itself, which is then printed, but she also sells the rights for translations and adaptions and thus the story. A translation is maybe the best example for the text-story difference: It's very much the same story, but it is obviously another text. Which is why I think translators earn a lot more respect than they are currently paid. Although they don't do the plotting and storytelling, they very much create an own text.
So what is a book? A book is first and foremost a physical object. You know the type: Lot's of paper bound together in a cover. That's all there is to it, really. But wait! What, in this line of thought, is an ebook? Isn't an ebook just a text and a story when intended, but it is not -- by our definition -- a book? Well, it does have the legal status of a book.
So, yeah. I guess we do need a different definition of what being a book really entails. Our ideas obviously have changed in this regard. So my proposition is the following:
A book is an economical unit in which text is sold, which entails a specific legal status.
So you don't really write a book. You write a text that tells a story. And when you're lucky, it's going to be sold as a book one day.