eInk achieves excellent battery life by being completely unpowered when the screen isn't changing. When you press the "next page" button, you're basically booting the whole device, having it render the next page, then shutting it off again. If you're using any sort of backlight (as "auto-brightness" suggests) then you won't get that benefit and I'm not surprised your battery life is more like that of a traditional tablet. The DX doesn't have a backlight so wouldn't have that problem. If you turn your backlight off then you should see similar results.