Published: October 1991
Publisher: Minstrel Books Paperback (1991-2000); Aladdin Digital Edition (2014-Present)
Plot: The Hardy Boys meetStar Wars at a sci-fi convention and things go out of control!
Review: Yes, this Mystery Stories book is available digitally. And as of this writing, Amazon.com's cheapest paperback listing is $769 USD (there are cheaper ones on other sites.
I remember the most unrealistic thing about this story was someone dodging a bullet after the trigger was pulled.
That's not unrealistic if it's Superman- he's faster than a speeding bullet, after all. :) 8)
It's funny, but The Secret of Sigma Seven is feeling very much like a Casefile: specifically The Last Laugh. I wonder if it was meant for the Casefiles but then reworked for the Mystery Stories.
Now then one plot element in this book, while still relevant today is also outdated today. Bootleg videos! Sure people still make bootlegs, but instead of selling physical media, a lot of times you can find it online!