Hi. I assume this has already been said, but just in case:
1. Reading. You can use Sigil, Stanza, Adobe Digital Editions (ADE), or Calibre to read iBooks. The iBooks are stored in ~/Music/iTunes/iTunes Music/Books/ (each of these is a folder inside a folder inside your home folder). Just install one of the above programs and double-click your ebook. If the ebook is DRM-protected, you
might be able to read it with ADE- I've not tried this yet. Most of my epubs are Gutenberg editions from Feedbooks.com or Manybooks.net. Those epubs are out-of-copyright and have no DRM.
2. Future readability. Epubs are an open content format (unless DRM'ed). They're basically zip files of websites - just a folder full of html files and images. So they are good for archival purposes. As for claiming that Kindle is the way to go, I disagree. It's completely closed. And I think Amazon is more likely to lose to Apple, given how Apple has taken the market with iOS devices. And not only is Amazon's format closed, but it has the same threat over it that iBooks does: Amazon can delete files off your device and their files are DRM'ed. At least SOME epubs - the ones worth reading - aren't.