[link|http://www.amazon.com/gp/product/0881792063/sr=8-1/qid=1155595044/ref=pd_bbs_1/103-5610957-5315005?ie=UTF8|The Elements of Typographic Style], by Robert Bringhurst. It contains an extensive discussion on this... and he's fully in agreement with Peter, even with monospaced fonts. It's a great book; I'd recommend it in general, not just for checking out the question of full stop double space.