I'm sure that someone here who actually has an after market book light for their Kindle 3 will post in this thread. Otherwise, maybe Amazon's lighted cover is indeed the way to go.
I bought the lighted cover even before I ordered the kindle, based on reviews here.
Still waiting for the k3, but the cover is here, and its really an improvement over the original. Very lightweight, even with the light, feels nice and the light is nicely built in, no bulk.
Have you looked at the neat pics?