The Nook itself has a 1530 mAH battery.
It depends on how hard core you want to get. Picked at random:
You could build something with D cells and put in a little switching regulator.
When you have the 5.0 Volts all you need to do is splice the power in, feed a hub or back-feed a hub.
In any case, don't blow your Nook up!
There are two possible external power modes on the Nook.
- It can takes all the power from outside and leaves the battery alone.
- It can charge and operate from external power.