There's a discussion thread on the Kindleboards about the use of hyphens, en-dashes, and em-dashes, and several of the people who posted clearly do not understand punctuation and typography. Here's what I posted (or attempted to post; it hasn't shown up yet):
The comments in this thread are indicative of why self-publishing gets a bad rap.
Failing to properly use the tools of the trade (in this case hyphens, en-dashes, and em-dashes) announces to a knowledgeable reader that the author/publisher lacks the ability to do a professional job.
It's like calling a plumber to unstop your toilet. Instead of bringing a plunger and a pipe wrench, he shows up with a table saw and a tire swing. He might still get the job done, but you'll know he's not a professional.
Although there are several style books available, the two most commonly used are The Chicago Manual of Style (primarily used by book publishers) and The Associated Press Stylebook (primarily used by newspaper and magazine publishers). Buy them. Read them. Keep them next to your computer and reference them often.
The 15th edition of The Chicago Manual of Style, which I believe is the most current edition, spends five pages explaining the difference between the hyphen, the en-dash, and the em-dash. The Associated Press Stylebook offers a much shorter explanation of the use of the dash, but the two agree about what constitutes proper usage.
Their key point of disagreement? The Associate Press Stylebook advocates the use of a space on each side of the em-dash when used within text and The Chicago Manual of Style advocates against the use of a space on each side of the em-dash when used within text. And if you know anything about typography, you'll understand why they differ on this point.