6502 Assembly Language Programming by Leventhal. This is how I learned. This book is great. I have the worn paper copy sitting 6 feet behind me as I type this. It's worth getting a paper copy if that is still possible. https://archive.org/details/6502-assembly-language-programmi... (https://archive.org/details/6502-assembly-language-programming)
6502 Assembly Language Programming
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