How did they succeed?
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.