i pondered this question several years ago when i set out to do my first eBook. there was a restriction and i think it it still a restriction that ebooks created with ibook author can only be used in the ibook store. i think that is still apple's policy and i have to wonder if i will need a team of lawyers if i exported to pdf format and tried to sell the book on amazon or any of the other ebook outlets.
so i took the approach that i would make the book so good that people would be willing to buy an iPad in order to have access to the book. i would think that if you only were going to put it on one type of device that you would choose an iPad.
on the other hand, i wonder if this approach is the correct one. i am about to publish my second ebook despite some serious challenges with the new ibook auth and keynote. i hope apple fixes the problem quickly, or else i will have to make some adjustments that will make the book not as good as it might be.
after i finish publishing the second book, i am going to take a serious look at an alternative approach that i have to think is better than pdf. it just makes sense for me to look at adobe's indesign, dare i say. it has a lot of interactive features, so i understand and can be used to make an ePub book that i have to think would be able to be used on lots of platforms. i am a big apple fan so i would think that i would still use ibook author even if i could get away with a develop once use it wherever alternative. i am sure that apple does not begrudge authors selling a book on different platforms because last i looked iTunes was available on a windows machine. i would point you in that direction.
still, it would be nice if (1) apple enhanced ibook author to be able to publish on other platforms and (2) quickly fix the problems with ibook author 2.3 or have better regretting testing. i understand that they are in the business of selling iPads though.
as an aside, my grandparents and many relatives on my mother's side are from michigan. mom was one of five sisters born in the farming community of oakley which is close to chesaning.