Since ebook readers are the perfect example of low-power-draw, long-term-use devices that benefit from solar power, why not put some solar cells on the back and make yours solar powered? This instructables shows you how.
The creator points out that this isn't relegated to just ebook readers, and you can technically slap some solar power onto similarly flat and wide-backed devices. The only question is whether your ebook reader will get enough power during reading from placing the solar panels on the back. If you hold it up with the reader side facing down, sure, but most people read with the e-ink side facing up, which means this would mostly be useful as a downtime charger.