My 15 year old musician son has developed a keen interest in the
circuitry behind analog and digital synthetizers, and wants to build
his first component for a custom synth, a VCO board. We have the PCB,
I'm gathering the components, and have given him an intro to
electricity and electronics, what the components are and what they do,
but I need a good book that, at a junior high school level, explains
all the concepts, and does so within the context of a fun read (dry
conceptual material won't get to first base with a 15 year old).

Suggestions?