Hi.
I just wanted to contribute to this forum by recommending an accessory I made myself to my Nook (I imagine it can be adapted for other readers)
It's just a rubber band and some hooks.
As you can see from the images, the hooks are placed in the gaps on top and bottom of the device. The rubber band holds them together and allows you to slide your hand or fingers. In this way you don't have to put pressure with your fingers to hold the reader. It is very confortable and gives you more ways and positions to read.
I definetely recommend it.
Here you see the band and the hooks (I guess you can change the threading of rubber band for other patterns)
In these pictures you can see I don't use my thumb to hold the reader.
Hope you find this useful!
PS: One last suggestion. I sandpapered the sharp glossy black edges of my Nook and now is much more confortable to hold.