In formal writing, the Em Dash (the type of dash being used here) is used to indicate a change in focus or structure within a sentence. There may be a short digression within a sentence:
"Yesterday, Sally—man, I hate that girl—told me my hair was unkempt."
In this sort of case, two dashes are used: one at the beginning of the digression and one at the end of it. There are also cases where the sentence never quite gets back on track:
"I was at the Bargain Mart when—don't you know the Bargain Mart?"
In these cases, there is only one dash, since the sentence never returns to it's original topic.
The em dash can also be used to indicate speech being cut off, as in dialogue in fiction novels.
"I had this great dream! It was—"
"Don't even talk about it," Jake interrupted harshly.
In the sentence you've presented, the em dash is being used to indicate a change in the structure of the sentence. The second part of the sentence elaborates on the proposed category of "genre fiction". Parentheses or a colon would also have served this purpose, but the writer in this case opted for the em dash.
Besides the Em Dash, there are four other dashes in the English language:
- The Hyphen
- The En Dash
- The 2-Em Dash
- The 3-Em Dash
I won't discuss those here (it would take me pages), but at least now you can understand the em dash, which along with the hyphen is one of the most utilized.
According to The Chicago Manual of Style, sixteenth edition (2010), it is acceptable to position a question mark immediately in front of a closing em dash. Here is Chicago's entry on this topic:
6.87 Em dashes with other punctuation. In modern usage, a question mark or an exclamation point—but never a comma, a colon, or a semicolon, end rarely a period (see 14.46)—may precede an em dash.
Without further warning—but what could we have done to stop her?—she left the plant, determined to stop the union in its tracks.
The Oxford Guide to Style (2002), at 5.8.1, concurs:
The question mark can be followed with a dash where necessary:
He left—would you believe it?—immediately after the ball.
And Words into Type, third edition (1974), without referring explicitly to dashes, also agrees:
Direct questions. Use a question mark after a direct question.
[Relevant example:] Nor was it it disclosed—why need it have been?—that John had taken the case.
The three example sentences cited above are syntactically indistinguishable from your example:
If your cousin invites you to visit—and why would he not?—I will expect your return after only nine days.
So multiple style guides view this form of punctuation as lying within the pale of acceptable usage.
Best Answer
When I was learning typography—many years ago--the convention was that you did NOT use spaces around en- or em-dashes. If the type designer wants extra space for readability or aesthetics, they will build it into the slug for the type (or in more modern cases, program it into the font).
Because of the proliferation of letter-spaced fonts online, where there is no allowance for a typographer to make a character with a little extra space for aesthetics, the logic for the standard doesn't exactly apply anymore.
But that's still how I do it. And as you can see, when dealing with the internet I tend to use double hyphens instead of em-dashes because you never know how a text editor is going to handle an em-dash, and those that handle them at all will often convert a double hyphen into an em-dash.
And as far as I know, substituting a spaced en-dash where you should use an unspaced em-dash is definitely out.