Apple has been adding 1M iBookstore users per week

Earlier this week, Apple iBooks chief Keith Moerer announced that the company has been gaining about one million iBookstore users per week since the release of OS X Yosemite. The announcement came at the Digital Book World Conference in New York City.

The iBookstore comes pre-installed on Yosemite, which likely help customers find it. I'd also suggest that, as Moerer mentioned, the large-screen iPhone 6 and 6 plus make for a more pleasant reading experience.