Okay, I've harped on the sales figure topic before, so perhaps I should stop. But Apple screwed up here. They entered the ebook market a year too late.
What Apple should have done was buy Stanza in early 2009. That should have been their first move into ebooks. You might not remember, but there was an incredible buzz around Stanza at that time. It was the hot new iPhone app. At the time Amazon bought the company, Stanza was the killer app. And in case you were wondering, yes, I'm pretty sure Amazon bought Stanza just to keep it out of Apple's hands.
So at this point I would argue that Apple will be more than a niche player in the ebook market, and i wonder if they should have entered at all?
If they'd never released iBooks (we wouldn't have agency pricing, for one), Apple could have simply stood back and said "sure we're in ebooks - look at all the ebook apps on our platform." Or Apple could have releases a completely free app with no ebookstore component, and then politely declined to enter the fray. They would have lost money on it, yes. But compared to Apple's usual profit margin, they're not making all that much of iBooks anyway.
As i sit back and look at the lack of data on iBooks, I have to wonder why Apple got into the ebook market this late. It has the overwhelming sense that they entered not because they could change the market, but because everyone else was already there. This is possibly an example of "me, too"-ism, taken to an unfortunate extreme.