I have books from every store on that list.
A good portion of them are/were freebies -- I like to have books for the apps, but I'm not necessarily going to spend money to put them there.
I haven't given any money to iBooks, B&N or Google, and I try not to buy books from Amazon (I have spent money on only one so far -- a kids' book to try on my son during the recent "Big Sale". He hasn't looked at it yet and I actually regret buying it.)