Of these short stories, only "Cold Case" was both a supernatural story and a 'fair' mystery, meaning one in which the reader has clues sufficient to solve the puzzle but isn't likely to. didn't use much otherworldly material, and didn't overexplain it. (And it's just a neat, spine-chilling little ghost story.)
Of the rest, some are successful because they reuse the background from longer books. , e.g., has much fun with pseudo-academic footnotes pointing out what she hasn't explained; she also has a classic ghost-story ending. "Doppelgangster" () lives up to its silly name. But mostly the stories were too short for the idea: so much of the 'magic' had to be explained to make the 'mystery' comprehensible that there was a constant rumble of stage-machinery coming on and off set, and no time for misdirection.
ISBN: 0446679623
So wrote clew in SF&F.