I have the M-Edge e-Luminator2 Booklight for Amazon Kindle
, which works well with my Oberon Kindle cover. Without a cover, it's useless because it has no clip-on means. The nicest thing is the weight. It only uses one AAA battery, so it's nearly as light as a feather. If you use it a lot, you'll have to change the battery every few days, but I imagine it uses no greater total number of batteries than other devices which use two or three cells.
I don't have a cover yet for my Sony Touch, and Olberon has discontinued their Sony covers, so I find I could also use a recommendation for a good light for the PRS-600.