I think the idea was to show all you could do with a handful of common and cheap parts. Electronics wasn't a cheap hobby back in the 70's. As a kid, I had to make do with what I could salvage. I was thrilled to find any circuits to build with parts I could get for free. Wasn't a huge Radio Shack fan back then, and still won't go to one as an adult. The parts were of the cheapest quality ( remember, I'm comparing to what I salvaged), and the prices were just way beyond what I could afford. So, as a basic introduction to electronics, with little or no money, I'd guess those books served a purpose other then just profits.