mwillega said:

I always recommend the MOS Technology hardware and software manuals as good starting points for understanding the 6502. Never seen anything better, when it comes to describing how the 6502 works and how to program it. Those two books are all I needed back in the 70s to come up to speed on 6502 assembly programming. Your mileage may vary.

I confess to limited direct experience -- my only experience with teaching someone assembly language was teaching _myself_ -- but as far as I'm concerned, the late Jim Butterfield's books would be very hard to outdo. They're written on the assumption that the reader is using an 8-bit Commodore, with some insightful leveraging thereof to simplify tutelage, but the explanations of how and why each assembly-language instruction works are superlative. G.