I've tried less than 10 lights before I settled for Zelco Max.5 (which I can't praise enough) for my Sony PRS-505 and in terms of the light and battery consumption it's great because it's one of the very few reading lights which employ the modern tiny LED (and not those dome-shaped older LEDs). However I can't clip it to the nook since it doesn't come with the cover like PRS did.
If you can hold off for a little while until when the cheap covers for nook flood the market I'd highly recommend Zelco Max.5.