Apple is now trying to enforce an ignored but apparently existing rule that specifically targets publishers of e-book apps offered in its App Store.

In a nutshell, if the publisher gives customers the ability to buy books outside of the app, such as through its Web site, then it must also offer the ability to buy books within the app itself using Apple's in-app purchase system. Otherwise, that app would be rejected.

Of course, if the books are purchased within the app, Apple gets its standard 30 percent cut.

"We have not changed our developer terms or guidelines. We are now requiring that if an app offers customers the ability to purchase books outside of the app, that the same option is also available to customers from within the app," Trudy Muller, an Apple spokeswoman, said in a statement on Tuesday sent to The New York Times and other news sources.

The move to strictly apply these guidelines follows Apple's rejection of the Sony Reader app on Monday on the grounds that it would have let customers buy e-books sold at the Sony Reader Store, thereby circumventing the App Store. If tightly enforced, these rules would clearly affect a variety of e-book app publishers, notably Amazon, which sells an iPhone Kindle app that directs its users to buy books at its Kindle Store.

Allowing Amazon's app to sell books through its Kindle Store, yet rejecting Sony's app for similar reasons could certainly be seen as a double standard in adherence to the guidelines. The move also ultimately leaves e-book sellers in a quandary. Do they give in to Apple's guidelines and its 30 percent cut as the cost of doing business in the App Store

It's "highly unlikely," says the Times, that Amazon, Sony, and other companies would share their e-book sales and customer information with Apple. That's especially true since Apple is not just a marketplace but also a competitor, selling books through its own iBookstore.