Quite right according to http://www.apple.com/ibooks-au...
Only available for iOS & Mac at the moment.
Sorry to have assumed that, like iTunes, Apple would open the platform.
Contract terms are worth a close look. I've heard vague suggestions that both Apple and Amazon want exclusive access to your work; also that they want to dictate prices on their platform and elsewhere. (You can't sell elsewhere at a lower price...) But again, that is rumor and I can't find time to dig in for actual facts. OTOH, an open textbook such as the OP proposes might be free so price is not a concern.
Other concerns involve updates. There are indications that newer versions of iOS and Mac OS are not working well with iBooks and iBooks Author. The book Yellow Submarine (from 2010) is no longer listed at the iBook store. Another concern, if you are using a 16GB iDevice, is that something fancy like Yellow Submarine can take 370MB of space.
iBooks seem to be the best example of what is becoming possible with living, interactive books. They are in the same 'bleeding edge' category with wearable VR devices and 3D 4K video. If you can't afford the equipment (the Apple tax), you're out of luck.
In the end however, if you want your book to have magical properties, to be able to sing and dance in 17 dimensions, your only choice is iBooks. The Yellow Submarine book was an example of what is possible when creative people care about the user experience.