The only power source a book needs is you. If you have to leave for a few minutes you have not lost the story. It is waiting for you when you return. You can pick up a book and resume reading at any time, after a few minutes, a few days, even a few years. A television picture or a movie might be lost forever, but your book is waiting.
Books are the perfect Time Machine. By the simple act of opening a book you can, in an instant, be travelling up a jungle river without once being bitten by mosquitoes, or you can almost die of thirst in the desert while holding a cold drink in your hand, or dine in the finest restaurants and never have to worry about paying the bill, or ride the wild country of our western frontier and never worry about losing your scalp to a raiding party.