If necessary, the battery can be replaced. A replacement battery must be of the following type:
Barnes & Noble Lithium Polymer battery
Model Number BNRB1530
Rating: 3.7V, 1530mAh, 5.66Wh
You will need a small Phillips head screw driver (size 0 or 00).
I recently picked up a 1st generation Nook for $20. I was a little weary of the age of the battery (6 or so years old?), but it turns out that this Nook is one of the few consumer electronics released in recent memory that was built to be repairable. (The battery was meant to be replaced! There are even official instructions.) This is in stark contrast to my 2nd generation Amazon Kindle which had hardware problems and became unusable right after the warranty period ended. That experience soured me on the Kindle, understandably.
At any rate, the only Nook that has a user-replaceable battery is the 1st generation model. Also, it seems that Barnes and Noble doesn’t seem to sell the replacement anymore. (I could only find one outdated listing). Fortunately, the batteries are easily found online for about $10. The steps are pretty obvious to replace it: pop off the back with your fingers, unscrew a screw, put the new battery in, and then reverse the process.
Between the replaceable battery and the free 3G internet included with the device, hopefully these devices stay in use for quite a long time!