It's important to watch for careful wordings: 22% of <i>sales</i>, 5 million <i>downloads</i>.<p>As a single data point, I've downloaded about 20 free books (I've read <i>The Invisible Man</i> and I'm halfway through <i>Pinocchio</i>). The overlap of free books on Kindle vs iBooks is pretty high (mostly all from Project Gutenberg).<p>I think the iBooks ability to download a free sample (usually first 30 pages) is the killer difference. I've downloaded a few samples when I first heard about a book, and every time I open iBooks they're there reminding me about a book I wanted to read, that I can pay for when I'm ready. Maybe the Kindle device has a better "shopping list" type feature, but all Kindle book purchases on iPad have to go through the browser.<p>I think iBooks is a better service, but the huge Kindle catalog will help Amazon. For a single book, the programs are similar enough that I'd just buy it from the cheaper store.