Kasumi said:Easy6502 is probably the best pure asm reference: https://skilldrick.github.io/easy6502/
You can try the lessons right in your browser with no other setup, so it's great even to just test what happens with some quick things once you already know all the instructions.
digit2600 said:I would say if you can, start with easy 6502 to get acquainted, but an excellent primer I found is the one chronosv whipped up on YouTube. Just type in chronosv nesmaker asm tutorial.