It really depends. I've seen solar panels that would be able to over charge the device.
All panels provide voltage, even if the sun isn't shining directly on them.
I don't know to which extend, but it may be that as long as you're holding the device, the panel would get enough light from it's environment to charge the device.
The only thing you would not want to do is put it in full sunlight,or put it in it's sleeve.
I have a gameboy charger, it consists of 2 solar panels of 2,5".
I also have a solar panel cellphone charger the size of an iphone (3.5")
The cellphone charger always provides voltage, in any room with more light than office light; it also has a built in battery. you can buy them on dealextreme:
If you have a sleeve like this one:
or this one:
you could easily paste 2 solar panels on the outside of it, with a wire connected to the reading device!
panels like these ones: