It's a dark day for e-book readers on the iPad, no longer can you buy e-books directly from within Apple's iOS apps. From the New York Times:
But over the weekend, updates were issued for many e-reader apps, removing links to their bookstores in order to comply with Apple's new rules. These stipulate that Apple receive a 30% cut from in-app purchases and subscriptions, something that many publishers balked at, contending that that cut was too high.
Apps affected include Kobo, Borders, Nook Kids and Kindle. You can still read e-books you've purchased from within the apps, but to buy new e-books you'll have to go to their websites via mobile Safari.