Originally Posted by Fbone
The last figure I remember from Apple was 180,000,000 iBooks downloaded (not purchased) during the first 9 months of 2011. They may have ended the year with 240 million, if rate continued. If Apple has less than 5% that would mean over 6 billion ebooks downloaded last year. Possible, I suppose. Or more likely Apple has a greater than 5% share and when you add in Sony, Google, BoB, Diesel, Smashwords and the others it's a higher probability. That throws off the other figures.
The OP asked about ereaders so the above doesn't answer or help him any.
As an iPad and iPhone owner, I would question whether Apple has even 5% of the paid e-book market. When iBooks came out, I downloaded the free Alice in Wonderland (on my phone and iPad), and when iBooks 2 came out, I downloaded the free textbook. But I haven't actually bought any books from Apple, and I suspect a lot of people haven't.