When you see people flipping through e-books on iPads or iPhones, there is a good chance they bought the books through Amazon.com, not Apple.

Now, Apple wants to change that.

Apple confirmed Tuesday that it would require app developers that sell e-books outside of their iPad and iPhone apps  through a Web site, for example  to also sell the books inside those apps. And purchases that originate in the app must be made through Apple, which keeps a 30 percent cut.

Apple said it was newly enforcing an existing rule, one that will force Amazon, Sony, Google and other e-book sellers to change the way their apps work.

“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.