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.

"Books are the original insulator. A shelf of books along an outside wall works well to prevent heat escaping,"

Link (Thanks, wiseGEEK!)