Various sources (such as Wikipedia) say that underlining words is a practice originally from handwritten documents, intended to show the printer that the words needed to be emphasized (with italics or some such). Today, I sometimes see underlines showing up in books and other printed material. When did books start using underlined text for emphasis?
Edit: Many of the answers and comments here seem to have a No True Scotsman problem. I'm well aware that good typography doesn't use underlines for emphasis, but plenty of books use lousy typography.