So what exactly is a book ?

With all the current hype in the electronic world I am constantly reconsidering what it is that I define as a book and measuring that against what others think also. The traditional view of something made from paper and bound between a couple of boards is fast going out the window as Matthew Ingram has described this tech site:
 http://gigaom.com/2011/04/22/what-is-a-book-the-definition-continues-to-blur/