As a typesetter (and the Senior Creative Graphic Designer at my company) I disagree with the assertion that there should be no no space on either side of the em dash. The whole point of typesetting is readability, and it is definitely more readable with spaces on either side. It also makes the typesetters job more doable because all professional typesetting programs force the words on wither side of the em dash to stay together on the same line, if there are no spaces on either side. This makes for ugly looking rag on left justified text, and awkwardly tracked lines when using full justification. The only alternative is to manually put in soft returns, and that is just not possible on a 456 page book on a tight schedule.