[Antinoos]

@Ufw: I'd strongly suggest to jailbreak your device. Turning the light off completely does not require any modifications to the system itself besides the jb. Just install KUAL and Vlasoftsoft's CoolReader port for Kindles (a freeware addon part of his shareware PBChess app).

Then do it like me: Only use CoolReader for your sideloaded books. (Believe me, this is definitely world's best eBook reading app if it comes to configurability and other useful features...) Inside CR you can switch light off completely - and the device remembers it even if you leave CR and use the standard Amazon software. Only a restart switches light on again...