Back in February, Apple began enforcing App Store rules regarding requiring in-app purchases of e-books in addition having options outside the app. Sadly, today that has caused the people behind iFlowReader to shutter their business because "Apple is giving us the boot by making it financially impossible for us to survive." Users of the e-book reader app are being warned to back up their data carefully, or they might lose access to the content they have purchased through it.
Previously, apps like iFlow Reader were able to sell content for the app only via non-App Store purchases. For example, with Amazon's Kindle app, you can use the Mobile Safari web browser to purchase books via your Amazon account. Apple doesn't take its usual 30% cut of these transactions. The new rules mean that developers wouldn't be allowed to do this. All such business would have to go via the in-app purchase API, using the user's iTunes account and with Apple taking 30% of the money. Existing apps apparently were given until the end of June to change how they work or face removal from the store.
UPDATE: Companies are not required to only offer in-app purchasing, but they are required to offer it as an option and, according to section 11.13 of the T&C, must price the in-app purchase the same as or lower than purchases made outside the app. In effect, this does mean that most purchases are likely to move to the in-app model and incur the 30% surcharge. Customers will presumably prefer to carry out the smallest number of steps to make the purchase, which is the in-app model.
I speculated in February that Apple's change could cause some popular apps to flee the store. We know that Sony's Reader app was rejected, that Readability had to shelve its native app (it later released a web app) and that TinyGrab also abandoned plans for an iOS app.