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There is an excellent book that covers the use of em- vs. en-dash vs.
hyphen. The book is called "The PC is not a Typewriter" (possibly "Your
PC..."). My book is packed with many others several miles away so I can't
look up the specific rules for you. I also think I've seen that information
in grammar books but I can't be sure. BUT, your typesetter may have been
correct to change the hyphens. I know en-dashes are recommended for use in
expressing mathematical ranges, for example 10[en-dash]20 typographical
mistakes per page.
Almost every style guide I've seen has said that neither em- nor en-dashes
should surrounded by spaces, but I've seen it both ways.
I can't wait to read the Saffire article. I've noticed my own writing hashed
with lots of em-dashes, especially e-mails.