What would be the scheme you recommend to use for an iOS app? Thanks.
I don't know of the specific DRM api for iOS ebook's, you should register for an apple developer account (free) and check their documentation.
I don't know if iOS has a default ereader app, it does support kindle, amazon's reader, which has it's own proprierty api. There are also standard ebook formats with DRM.
Are you trying to write a ebook reader, or publish an ebook on iOS?