6502 Machine Language for Beginners
savetz writes "If you've always wanted to learn 6502 assembly language, now's your chance. The full text of the classic, best-selling 1983 book Machine Language for Beginners is now on the Web. It includes examples and program code for Atari 8-bit, Apple ][, PET/CBM, VIC-20, and Commodore 64 computers."
An old eight bit processor is the best tool for understanding how computers work. They are simple enough to avoid discouraging the student, but the core concepts are still the same today. There are free simulators, so students don't have to put up with uncomfortable data entry or long loading times.