The graph shown above was taken from a site that discusses the impact of e-book sales on traditional books. The data that you see on the graph was collected by PricewaterhouseCoopers (PwC) and published in article that they wrote. It shows the expected time when ebook sales will overtake the printed book, as soon as 2017. This raises some questions. In three short years, will we start to see printed book start to be replaced all together instead of just co-exsiting along side the e-book? There's not sure way to tell, but judging by the huge upswings in e-book sales that we've seen, it seems the the traditional book has a lot of catching up to do.
Sources: http://www.statista.com/chart/1159/ebook-sales-to-surpass-printed-book-sales-in-2017/