Randomex – I think I was thinking of Devora Doresh. However, I’m not familiar enough with any of these books to be able to compare Devora Doresh to Gemarakup. I had no idea that in one the readers are asked to solve the mystery, and not in the other.
I guess that Gemarakup is more similar to Encyclopedia Brown, which I am familiar with, since unlike the others, it was around in my childhood.