The short version is "Take our EPUB file and upload it to the iBookstore".
We generate PDF, EPUB and MOBI files for you, whenever you preview or publish a new version of your book. Anyone who buys your book from Leanpub can download the EPUB file and open it in Apple's Books app.
As an author, you can also take the EPUB file and put it up for sale using Apple's iBooks service.
If you would like to sell your Leanpub-generated EPUB file on Apple's service, here is the best place to get started:
The #1 goal for us with EPUB generation is to ensure our EPUB files work on iPhone and iPad (either in the Leanpub app or in iBooks). The #2 goal is to ensure they can be sold by authors in the iBookstore with no issues.
We are developers and do not believe in lock-in. We want you to use Leanpub because we are the best. We charge money and have a 45-day refund period because we are confident our service is worth it.