I strongly recommend this one
if you're looking for an external speaker. I own one myself and I would buy it again anytime.
For the price, I don't find anything to complain about. Build quaility is excellent, volume is insane and I even like the SQ. There is actually something that could be called bass coming from this thing.
The mp3-player feature can be ignored, it can't even shuffle or skip albums, but you might like the option to use it as a card reader.
It can be powered with 2 AA batteries or USB. Haven't tried to power it via USB host on the Nook but it probably won't provide enough juice.
a good review.