Amazon may be the leading e-book publisher out there, but Apple's iBook platform is threatening to unravel its dominance.

Since Apple released iOS 8 in September, the company has added a million new iBooks customers every week, according to Apple's iBooks director Keith Moerer.

Apple's decision to pre-install iBooks on both iOS 8 and OS X Yosemite has contributed to its tremendous growth.

About 70% of Apple's mobile devices are using iOS 8 now.

Apple's phablet-sized iPhone 6 Plus may also be encouraging customers to try iBooks.

Moerer said at a conference on Thursday that the company was seeing more people downloading books on their phones since the latest iPhones came out.

This is some positive news for Apple's ambitions in e-books. In 2013, the company lost the first round of an antitrust lawsuit that alleged it conspired with publishers to fix the price of electronic books at a higher price than Amazon had been selling them for. A federal judge ended Apple's alleged price-fixing and forced the company to pay up to $450 million in a class-action settlement.

Apple is appealing that decision and thinks it's in the right.

In December, Apple exec Eddy Cue said he'd "do it again" in reference to Apple's iBooks strategy.