Since this is an old thread, but was a top entry off of Google (meaning, it's likely that what you're looking for, is certainly posted), I thought I'd add the following for posterity:
Screen Brightness on a Kindle PW2:
[root@kindle max77696-bl]# echo 255 > /sys/class/backlight/max77696-bl/brightness
[root@kindle max77696-bl]# cat brightness
255
[root@kindle max77696-bl]# echo "Backlight is on. Full brightness. A byte value"
Backlight is on. Full brightness. A byte value
[root@kindle max77696-bl]# echo 0 > brightness
[root@kindle max77696-bl]#
See:
https://www.mobileread.com/forums/arc.../t-200266.html