I've looked into this myself a few times and the biggest issue with using an e-ink device is that you can't download an epub version of their books. You have to

I've looked into this myself a few times and the biggest issue with using an e-ink device is that you can't download an epub version of their books. You have to use their application to "view offline" and from what I have read from others experiences it's not that great. I decided to just use an iPad when I'm using their app.