Check out this great project from Instructables maker flapke -- it walks you through how to install solar panels into your e-reader, so that your device can run directly off sun power.
This hack is definitely for people who feel comfortable messing around with electronics. If you don't have a little experience under your belt then you might want to try this hack on an e-reader you aren't particularly attached to.
To accomplish this modification, you'll need:
an e-reader (this one used a Kobo but the maker is sure it can work with other devices)
solar panels (he recommends 0.44W Si Solar Cell Panel (5.5V 80mA))
A Schottky diode (which was updated to a MAX1555 Li-Ion cell charging IC during modifications)
A MAX1551 or MAX1555 IC
A SOT23 to DIP breakout board.
And a few other small components including a small capacitor, wires, a soldering iron and solder, a dremel and padded double sided tape.
The steps are fairly simple, and include taking apart the e-reader cover and cutting a hole in the back portion. Then you wire together the solar panels and install them on the cover.
Solder the MAX1551/1555 to the breakout board, wire up your solar panels to the battery, put the case back together, and voila!
Of course, it's not quite as simple as that. There is some careful work to be done when putting these components together, but the Instructable walks you through the important steps. You'll also want to read through the comments posted on the hack, which bring up a lot of interesting points on risks you may or may not want to take when doing this to your e-reader.
If you don't want to hook solar panels directly up to your battery on your e-reader, you could also use an external solar charger that sends juice to your e-reader when you want solar power. But the hack looks fun and interesting.