An Arduino
IS an AVR, basically a simple 'bare bones' system designed round an AVR processor with the required parts to make it useable, likewise the STM32 is available as an addon for the Arduino IDE, and simple ready made boards are available.
I would suggest you look at the Arduino schematic and STM32 (Blue Pill) schematics, like this:
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rarecomponents.com
It can't really get much simpler, and it's
VERY much faster and more capable than a standard Arduino.
As for books?, probably not much about for what you're looking for, but loads of Arduino books.