Direct sunlight has illuminance of about 50,000 lux. A typical indoor home or office is around 500 lux, or about 1% of the light power. So a 6" Wysips panel that produces 500mW in full sun would only produce 5mW indoors.
I don't know about the Bookeen e-reader, but according to
this teardown, the battery in the Kindle Voyage has a capacity of 5Wh. Amazon claims a Voyage charge is good for 6 weeks of reading at 1/2 hour per day (21 hours total) with wireless off. Given the 5Wh battery this indicates an average power consumption of 240mW.
Using a Wysips panel would charge the Voyage about 10% per hour in full sunlight when off or 5% per hour when reading. It would only charge about 1% per day indoors even with the device left off.
I mostly read indoors, so for me this technology wouldn't be worth the added cost. I would much prefer wireless charging so that I could have a charging mat on my nightstand to charge overnight after I finish reading in bed without any need to attach a charging cable.