Books on Electronics -
Books on electronics are useful, especially the ones that are free.
Electronics Books on Wikibooks:
https://en.wikibooks.org/wiki/Practical_Electronics/Amplifiers
Books on Amplifiers
http://diyaudioprojects.com/diy-audio/diy-audio-books.htm
https://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/
DIY Instructions
https://www.instructables.com/Make-your-first-Serious-Amplifier/
https://www.build-electronic-circuits.com/build-your-own-amplifier/