I really like this:
http://www.amazon.com/17228-Lights-B.../dp/B002JLWVXW
I have the blue one to match my reader. I bought mine at Wal-Mart for $4. Still on original set of batteries. Clips on to the back cover, folds flat, perfect!
I DID have to put a small piece of black electrical tape over the top to put a stop to some of the light diffusion.