Mostly these two: https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7 https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7 and https://gbdev.io/pandocs/ https://gbdev.io/pandocs/

Mostly these two: https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7 https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7 and https://gbdev.io/pandocs/ https://gbdev.io/pandocs/ It's incredibly well documented !