I like the Z-light (3 LEDs) for portability and lighting performance. Costs about $15 at B&N. Very thin, disappears in a pocket. Uses two of those large disc/button batteries. I bought a bunch on eBay cheap,haven't used up the first set yet.
One general tip: for more uniform lighting, clip the portable booklight on the *side* of the Reader (I clip to the folded-back cover) instead of the top.