In a majestically depressing graph, which you can check out here, we find that, while book related apps account for 18% of the apps in the iPhone app store, they only account for 3% of downloads. And, as we learned from the whole "Kindle bestsellers" thing, downloads do not mean purchases.
The caveats here are, of course, 1) that these stats were mostly collected prior to the launch of the iPad and thus Apple's bookstore, 2) that the Amazon app is a single app that you can use to download many books, and 3) these stats are crowdsourced, not official from Apple, so the potential for discrepancies are huge.
That said, I think the only right thing to do is panic about the demise of the industry. Woe to all of us, the book is dead.