UPDATE:
We heard from a customer that--to our stunned disbelief--with the brand-new Amazon Fire Kindle, you are able to buy eBooks from a variety of sources (like Green Apple), not just from Amazon, as has been the case since Amazon's very first e-readers. We will confirm this and update our device advice soon. Here's the excellent app we recommend for your Nook, Kindle Fire, or other Android-based device--it's based on the popular BlueFire reading app, so it's top quality.
If this is true, you'll be able to read eBooks bought from any seller on that device, which could be very good news for Green Apple and its loyal customers, as most eBooks we sell are priced exactly the same as at other competitors. We'll be back soon with an update.
Super thanks to loyal Green Apple customer Lovestampmom for pointing this out.