I used Goodreads' ordering of the Rebus series to determine when I read this novella -- other sites might have led me to read this before Dead Souls, as it was published. I might have gotten more out of this book if I'd read it in that order, but it might have hurt the novel. I'm not sure.
Basically, this is one of the subplots of Dead Souls -- Rebus' looking for the missing son of a people he knew in school -- in its original form. It'd be modified, expanded, and given a different ending in the novel. There's a subplot, mildly related, involving organized crime and gambling -- in much the same way that other crimes were associated with the missing person's case in Dead Souls.
It is interesting to see how Rankin wrote something, and then came back a couple of years later and repurposed it. But that's about all I have to say for this. It was interesting -- but the version in the novel is better. The subplot didn't do much for me, either. It was okay, but it really didn't seem necessary.
The completist in me is glad I read it, but I think I'd have been okay with missing it, too.