Exponential charts are tricky. They can be used to minimize fast growth when it is quantitatively fast, just slow percentage YOY growth. That is the state we're in now. With ebooks at 27% market share (probably far better as that is comparing retail print versus wholesale ebooks), one would expect slower growth as order of magnitude would have ebooks at over 90% market share in no time.
Notice how fast the ebook growth was proportionately in 2009 and 2010? The market went from nothing to a significant market. In 2011, the publishers in the AAP's efforts to preserve print book growth worked until the holiday 2011 ereaders hit customer hands. Notice on the 2nd graph that sales are still accelerating faster than the 2010 liner trend line. So even if the exponential chart looks somewhat flat, it is still healthy growth.
Compare to the above trendline/prediction. Ebooks are slightly above the curve. My 50% market share prediction by 1/31/2013 might not be off by much. :)
Note: I still expect print books to hold on. There is a nostalgia with books. However, the economies of scale of print are starting to fall. In particular with MMPB.