The words em and en refer to actual typesetting measurements, and they’re dependent upon the point size of the font you’re using. For example, if your ebook is set in twelve-point Bookman Old Style, then an em space is twelve points, or the width of the Bookman capital letter M. An en space would be the width of the capital N.
Going by that same measurement, the em dash is one em wide and the en dash is one en wide — again, the width of the letters M and N, respectively.
When used in a book, em and en dashes have different meanings. For the most part, an en dash means “through,” and usually refers to dates. As an example, if you were trying to say “the sale runs from May 15 through May 17,” that would be typeset as “May 15–17,” with an en dash between the numbers.
(More on that sale in a moment.)
When properly typeset, there are no spaces around en dashes. The numbers and the en dash all butt up against each other in a nice, friendly, crowded way.
On the other hand, an em dash breaks a sentence for a parenthetical thought. Here’s an em dash used properly:
“Mike said — he’s my cousin — that we’re in danger.”
The longer em dash is used to break the sentence for the additional information about Mike, that he’s the speaker’s cousin. Also notice that the em dash is more standoffish and less friendly than the shorter en dash. This is personal preference on my part, and I set the em dash off this way, with a space on either side of it, because e-readers don’t hyphenate very well. Without those spaces, the em dash joins the two surrounding words in an unbreakable bond, and most e-readers will then dump the lot onto the next line, leaving a big blank spot in the middle of the paragraph. It looks ugly, to me, at least.
The exception to spacing out the em dash is when it occurs at the end of a sentence. There, I butt the em dash up against the last word, like this:
“My cousin Mike said—”
That way, the punctuation doesn't get broken off from the final word and dangle, all by itself, on the next line. That looks ugly, too.
Okay, now, about that sale. Certain Dingbat ebooks will be on sale May 15–17. The final list is not yet complete; we’ll announce that closer to the sale date.
One thing is certain — our new releases will NOT be on sale. J.L. Salter’s new romantic suspense novella, One Simple Favor, is scheduled for release toward the end of next week. And Rob Marsh’s silly juvenile picture book, The Inquisitive Guereza Monkey and the Post-Soviet East Bloc Abandoned Nuclear Power Plant, is now available wherever fine ebooks are sold!