Well, we already know the size of the market.
Here's a report on the latest update covering 2012, until nov, just as the Bowker report.
http://www.the-digital-reader.com/20...up-44-in-2012/
It works out to about $7 billion a year, of which 24% is known to be ebooks.
It doesn't take much math-slinging to figure out the rest.
Or the underlying trend.