That didn't take very long at all:

Digital books are now “the dominant single format” in the adult fiction category, according to a new BookStats report from the Association of American Publishers. eBooks exploded in the adult fiction category last year, accounting for 30 percent of net publisher sales in 2011–up from 13 percent the year before.

Are we even a generation away from people automatically thinking of e-books when you mention a book in casual conversation? When do we have to start calling them "print books" by default? I enjoy reading e-books, but I'm still not looking forward to that moment.