Easy 6502 - Learn the 6502 Assembly Language
It's fun to see people being encouraged to relive my youth. And 6502 assembly is indeed a nice, compact thing to learn.Though I think the most valuable thing I learned from studying assembly was actually about higher-level languages: How function calls work, and the mechanism of stack frames.
This discussion doesn't reach quite that far, and actually I'm not sure how one would work such things into a discussion of 6502 assembly – I learned about them in the context of 680x0 assembly. Back in my 6502 days, I never even encountered a compiler, so I never had the chance to disassemble a C or Pascal program. (To this day I can't name a 6502-based C compiler, though I'm sure they existed someplace. I knew about Pascal compilers, but I never got my hands on one; back in the 1980s compilers actually cost money.)