I wouldn't worry about ereader's battery life, but if really want one, I would suggest something like this
http://www.amazon.com/Duracell-Insta...8097847&sr=8-2
Most of the newer generation of eInk reader with pearl display with faster processors don't have user accesible replaceable battery to save weight, compact size and probably save manufacturing cost at the same time. You'll most likely be sacrificing the overall quality of the ereader for a one with replaceable battery. IMO it's not worth it.
If i were you I would also use something like an ipod nano(i use a 16GB one), creative zen, or sandisk sansa to listen to mp3s, they barely weight anything and you don't have to log around a giant ebook reader(comparably speaking) for just listening to mp3s.