PricewaterhouseCoopers recently asserted that e-book sales will catch up with hard cover and paperback sales -- at least in the UK -- within the next four years. The same source anticipated a substantial drop (more than 30%) for printed books during that same time period.
I suspect this prediction warrants an "unlikely" at best. Digital book sales actually appear to have reached a plateau recently; the trend no longer points skyward. Moreover, it is obvious that printed books are here to stay. Nevertheless, the PWC prognostication is noteworthy, even if somewhat dubious.