I once read a really good book about discrete RF circuits (made from transistors/diodes/...). It was composed of ~100 different circuits and explained what each part did. (you could not always easily split the design, because of numerous feedback loops) It was a lot of fun studying it. You'd learn to find oscillating circuits, circuits that used non-linear elements to shift signals between frequency bands, to identify buffers and amplifiers, LP,HP,BG,BP filters, etc.

Similar thing in digital world is trying to understand simple ICs like, operation amplifiers, TTL logic, SMPS control circuits at the low level.