Screen Filter app is the one I use, but there are many others available that might work. One thing I've noticed though is that the latest version of Screen Filter no longer has any manual settings. It used to allow you to set the amount of dimming to occur, now you just have to use what it comes with.

BTW, I think the readability of the text for books is way better on the Fire than on my Pad. The iPad is rather pathetic at rendering text and has a lower resolution.