iBooks only supports iOS. I don't use it myself. But from the looks of it, it's probably very similar to Newsstand, which is basically a special Apple directory made to look like an app, used to contain iOS-only content (i.e., other apps made to look like files).
So it's not really an app containing book files, but a directory containing apps. The implication of this means it is possible to transfer books (which are really more similar to apps than files) from one device to another for the same owner; but AFAIK
, not possible to transfer away from one owner to another (like you would with real paperbacks) or away from Apple's realm.
Still need my morning coffee, so hopefully my explanation is somewhat coherent.