Do transparent electronic paper displays exist as a commercial product?
Not likely. E-ink consists of an array of tiny balls between two two electrode sheets.



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A positive or negative electric field makes that either black or white pigment chips will be at the viewing side. If those were transparent you would see the inactive white chips through the black ones and vice versa, and have no contrast.  

There's an illuminated Kindle, the Paperwhite, but that has front-lighting, not back-lighting:



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