The older Harris Corporation AM and FM transmitter manuals was what came to mind when I wrote that. As equipment got modernized, there was less for the operator to know, so they get shorter and shorter over time. Look at the SX-1 AM manual under "Principles of Operation" to something like the HT-35 FM manual under the same.

Also.. early computer manufacturers like MITS had manuals in a similar vein for their Altair 8800 box but you can find many examples in this space, there's a stub of a Wikipedia page just for it:

https://en.wikipedia.org/wiki/Theory_of_operation