I think O'Reilly is fairly interesting because they were about the best case for non-DRM standards based ebooks. Their content was very time limited in value, customers were savvy, shouldn't be too price sensitive, and were 'saying' they wanted the unencumbered ebooks.

But the market didn't actually behave that way. They had trouble selling ebooks and piracy was a huge issue.

The next one to watch will be Tor. If they back off of their DRM free stance we'll know that its basically an untenable model.