What is a book?

This question has been haunting me since our first class discussion. What is a book?!?
The answer to this question matters. Definitions matter. They are the foundation of our understanding of things. They influence our expectations and direct our actions.  Defining the book is especailly relevant as new technologies in enhanced books blurs the lines of our traditional understanding of the book. As we work on creating our own interactive books, I think it's important to return to this question to help clarify our projects. In creating interactive narratives with all the exciting tools that technology offers, we may run the risk of creating a cool app or interactive text based game and losing the essence of the book.

I've posted some thoughts about the definition of the book below. I would love to continue this conversation throughout the semester as we create our books and see if our understanding changes. 

Merriam-Webster dictionary:
 a :  a set of written sheets of skin or paper or tablets of wood or ivory

b :  a set of written, printed, or blank sheets bound together into a volume

c :  a long written or printed literary composition

d :  a major division of a treatise or literary work
(* The dictionary has not yet accounted for the advent of ebooks and enhanced ebooks in its definition)

What is a Book? by Alexis C. Madrigal
"In the Kindle era, it seems pretty obvious. There is an implicit argument in the act of digitizing a book and removing it from the shelf: a book is its text. A book is a unique string of words, as good as its bits. 
But printed books are also objects, manufactured objects, owned objects, objects that have been marked by pencils and time and coffee cups and the oils from our skin. "A book is more than a bag of words," the project's founder, University of Virginia's Andrew Stauffer, told me. "These books as objects have a lot to tell us."
(* This is an interesting article. It discusses the BookTraces project and the physicality of books. There are some interesting ideas about how the social community connects to books in this article also.) 

What Is a Book? The Definition Continues to Blur by Mathew Ingram

"It used to be so easy to define what a book was: a collection of printed pages bound inside a cover (hard or soft) that you could place on a shelf in your library, or in a store. Now, there are e-books, and blogs that turn into books, and long pieces of journalism that are somewhere between magazine articles and short books — like the recent opus written by author John Krakauer, published through a new service called Byliner — and a whole series of ongoing attempts to reimagine the entire industry of writing and selling books. If you’re an author, it’s a time of incredible chaos, but also incredible opportunity."