I don't think I've ever come across a prelude to a novel I'm about to read before, so I was intrigued to give this a once-over when I read about it.
Short though it is, Case 501 really is exciting. It begins with the central duo finding themselves in a dark warehouse, and it quickly becomes apparent that they're not alone. Leggett builds the tension nicely and the description of how a certain part of the duo's high-tech kit effects them is particularly well done.
I suppose unavoidably, the prelude offers more questions than answers, which is effective, and subsequently I can't wait to start the main event!