My constant reference for state of art but very down to earth circuit design..
"Small Signal Audio Design" Douglas Self. Very little maths except dBs and he goes into CS,Rs, and active components very deeply. Some very useful circuits in it.
For valve work as applied to guitars and basses. "Tube Pre amps for Guitar and Bass" Merlin Blencowe. Bit more math but you don't have to understand it. Lots of very useful circuits and ideas for amp pre design. The section on FX loops is worth the cover price alone.
I am waiting for him to get around to git power amps! Don't think he has yet.
When I was young my mate had a Selmer little giant it was only like 5 watt or something but it had a great sound, It' a shame nobody makes little kits like that to build?,, all the small kits I've seen cost big!.