eReaders using eInk like the Kindle, Kobo, or Nook use very little power if you adjust a few settings. This will extend the time between recharging your e-reader battery and give you more time to read that new e-book.
Your Kindle can last for quite a few days on a charge. Still though, there are a few features that will quickly drain your battery down. 3g searching is one of the largest energy users on the Kindle. If you are in an area where the 3g network is spotty and comes and goes your Kindle will consistently seek out a better connection. This will pull a large amount of power from the battery.
WiFi searching is another power hog and when not needed it is best turned off to conserve power. To turn the wireless off, WiFi and 3g, just press Menu and then press Wireless Off. Also when not in use you can just turn the eReader completely off.