I'm not sure you will be happy with a lightwedge - I tried a Lightwedge-Imitation (cost 10€) that I'm sure is not the same quality as the original one but uses the same concept.
It works really well on Paper but the problem with the Cybook is that - the wedge being bigger than the screen itself - there is a distance between the wedge and the display so the screen doesn't get illuminated enough, the wedge shines (scratches shine especially bright) but the screen stays rather dark.
My guess is that you will be happier with a normal reading-lamp (it's cheaper aswell
)