And it is worth noting that Apple was rather pushing the record industry to go off DRM and they finally succeeded in it.
https://web.archive.org/web/20080517114107/http://www.apple....
Blame the MPAA here. Apple would rather have it DRM free. It's better for consumers, it's better for their sales.
I think that was the major tipping point for DRM on music. Apple followed after that.
But the book publishers would apparently be happy for ebooks to go away, Amazon to stop selling books, and their higher margin deals with Borders to come back. So they called Amazon's bluff.
The only DRM that should be on ebooks is there should only be one version of it at a time. Just like a printed book.
I don't know much about Apple & Amazon's timelines, as such things would be secrets.