The editorial staff of the UK's Bookseller magazine want you to know that lining your exterior walls with books is practically a moral imperative, since it reduces your energy costs and saves the world. You don't have to tell me -- I've been lining my walls with books for decades.
Link (Thanks, wiseGEEK!)
In today's environment-conscious world, they also have another valuable function. "Books are the original insulator. A shelf of books along an outside wall works well to prevent heat escaping," says Joel Rickett. "If all the books were removed from the homes in Britain, our energy bills would rocket."