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What Amazon Kindle? Here's an Open Source eBook Reader
(itsfoss.com)
This is a most excellent place for technology news and articles.
It looks like it is powered by a microcontroller. Maybe it isn't powerful enough to support epub?
It's a 120mhz Arm CPU. That's more than enough for epub. For comparison the 25 Mhz 68030 in the Next computer used Adobe Postcript (PDF) as it's GUI.