What is the battery life like on these electronic books like Nook and Kindle?

I'm thinking of getting one for use while wilderness camping. The idea is that I can load several books into it rather than stuff several pounds of books into my back pack and lug them miles into the woods. Could I expect to be able to use one for say, an hour or two a day for a week without the battery going flat too quickly? Also, are the batteries removable so I can bring a spare charged battery?