What makes a book?

Nearly every day on the subway I see ads for audible.com, selling audiobooks, of mostly pretty popular titles.  Some trains have ads all the way across, from one end to the other, like this:

But one thing it got me wondering is...  

What is a "book"?

I've always thought of books as paper with printed words or pictures or combinations, and more recently with ebooks, electronic versions of printed words and pictures and combinations.  But the key thing to me is that a book is words.

But these audio books, are they still books?  Where's the dividing line between a book and a performance?  How often would we go to a play and after it's finished say "wow, that was an amazing book!"?  We might say it's based on an amazing book, but we don't refer to the play as the book.

I've only ever listened one audiobook, a performance, definitely a performance, of The Hobbit, performed by one of my favorite actors, Nicol Williamson (more famous for playing Merlin in Excalibur). This is something done long before audiobooks were commonplace, so I don't know if modern ones are similar.  In it, he did all the narration and all the voices, giving each character a distinctly different voice.  I've listened to it a hundred times or more since my mother got the tapes in the late 70's.  Williamson has probably become my favorite actor simply because of how many times I listened to that through my child hood...

And I sometimes wonder what a modern audiobook might sound like.  But I expect to be disappointed if I ever listen to one, because I can't imagine many actors doing as good a job as Williamson did for my favorite story.

But back to the original question in my mind, I have no doubt that the one I have heard was a performance, not a book.  I never thought of it as a book, but only as a dramatic performance of a story.  And maybe that's part of why I don't want to listen to an audiobook, by calling it a book instead of a story or a performance, my expectations are set for disappointment.