I don't build synths. If I did, this is where I would start: https://archive.org/details/electronic-music-circuits https://archive.org/details/electronic-musi

I don't build synths. If I did, this is where I would start: https://archive.org/details/electronic-music-circuits https://archive.org/details/electronic-music-circuits There is a link to download the PDF below the viewer. Hard copies are more expensive than most books but less expensive than many analog synths. Good luck. *Edit* Came back to say, I would also consider Eurorack simply because that would simplify decisions about form factors, power supply voltages, manufacture, testing, etc. during design.