PocketBook 360 has an external easy to reach battery case. Many other readers you can just open up (with a bit of care) and replace the battery thataway. Reader batteries should last long - there is little to no heat, no high power consumption, and the device remains usable even if the battery loses 50% of its capacity.
Notebook batteries are much more prone to dying (constant heat, and they suck the battery dry in under an hour). My netbook battery didn't last a year - I'm not getting a replacement either, it still works with PSU...
Don't worry too much about it, get a device you really like instead.