The first time I saw one of these books was around 1991 in Saldanha library. I can't remember exactly which one it was - maybe "Practice Your Basic". I was around 10 and didn't even have a computer at the time, but was super interested in them.
In the late 90's, probably 1996 or 1997, I purchased "The Mystery of Silver Mountain" from a second-hand bookstore in Claremont. It might have been CAFDA Books. At the time I had an Acorn Archimedes A310 machine which no doubt wasn't quite one of the compatible machines, but I'm sure I made do.