Actually, it is probably yes on 11 (50-year down the track). Unless Apple encrypted the format, it is epub3 - zip file with html, xml, and other standard-based formats in there. Unzip, extract, modify, recreate under emulator. Same story as OpenOffice.org. Much better than than MSWord formats and - frankly - much better than books from 50 years ago, which if you wanted to do any "modern" thing with (e.g. searching), you would have to rip apart, scan, do text-recognition, layout-reconstruction, etc.
Now, if Apple has put a real DRM/Encryption in there, it would be quite another story. Even then, 50 years down the track, any digital modern encryption will probably be a 10-second crack away.
> Now, if Apple has put a real DRM/Encryption in there, it would be quite another story.
I'm pretty sure iBookStore ebooks are DRM'd (same as Kindle files) (EPUB has a provision specifically for DRM, FWIW) but according to an other commenter the .ibook output of iBook Author is an epub3 book, you can just change the file extension and drop it into an ebook reader (though Apple has apparently added e.g. CSS extensions for nifty effects, which your ebook reader likely won't support).
Now, if Apple has put a real DRM/Encryption in there, it would be quite another story. Even then, 50 years down the track, any digital modern encryption will probably be a 10-second crack away.