Mike Shatzkin: from some perspectives, we are tipping right now and publishers’ metrics will show it:

I have heard the argument from very smart people that ebook adoption will plateau at some point. Since it has been doubling or more for the past three years and was often placed in the mid-teens for new fiction and narrative non-fiction by the last quarter of 2010, we know that it can’t continue to double for the next three years without exceeding 100%. Nonetheless, predictions that ebook sales would achieve 50% in the next five years and that bookstore shelf space would drop by 50% in the next five years — which is what I thought would be the case — seemed pretty aggressive six months ago.

They don’t seem aggressive anymore.