A solar charger is entirely possible. The Ebook takes a DC input. I think it's 12V at 1 Amp, so all you need is a solar charger that outputs 12V with 1 or more amp capacity and you're set. The connector is sold at electronics stores in a "multicharger" type of product. you take that connector out and solder it on the wires and voila you're set.
That's a great idea. I'll look into the cost of that.