Originally Posted by JSWolf
You can subtract Apple's walled garden because a lot of it isn't standard ePub. But you have to include B&N's walled garden as their (no-fixed reflowable) ePub is ePub. It just has a different DRM. So yes, that does count as ePub because it is.
Can you readily get the Epubs out of B&N's ecosystem? I was under the impression you cannot. If that is true then B&N's wall is as high as Apple's wall. Both must be excluded.
And if you're going to make a claim that "a lot of it (iBooks) isn't standard ePub" then you need to back it up. Got figures?