dashes

[Vertigo]

I think you might be right that it is changing - though I'#m not so sure it's gone as far as preferred. Certainly in older books I've read (Wells, Conan Doyle, Verne, Wodehouse etc.) it always seems to appear with no spaces whether it is paired or not. In more modern books I have seen paired dashes appearing both with and without; not too sure which I see more often though.

I would add though that the same web page I quoted above goes on to say:

While there are many more possible uses of the em dash, by not providing additional examples, I am hoping to curb your temptation to employ this convenient but overused punctuation mark.