Mike is smart, very smart this paragraph nails the problems of booksellers and publishers too:
One distracting fact for analysts considering this question has been the apparent slowdown in the growth of ebook sales, suggesting that there are persistent print readers who just won’t make the switch. The encouraging fact is distracting because it is incomplete as far as predicting the future of shelf space at retail, which is the existential question for the publishers, wholesalers, and bookstores (and, therefore, by extension, for legacy authors too). We need to know about changes in the division of those sales between online and offline to really have a complete picture. If ebook takeup slows down but the online buying shift doesn’t, the bookstores are still going to feel pain.