A little bit of Easy 6502 [0] but mostly Richard Haskell's Apple II-6502 Assembly Language Tutor [1][2] along with Virtual][ [3] . Basically I'm trying to use assorted manuals I could find in my childhood home's basement, most of which seem to be out of print (although there are some books floating around on archive.org).

[0] https://skilldrick.github.io/easy6502/

[1] https://www.amazon.com/Apple-II-6502-assembly-language-tutor...

[2] https://archive.org/details/Apple_II_6502_Assembly_Language_...

[3] http://www.virtualii.com/

Richard Haskell's book ( http://www.amazon.com/Apple-II-6502-assembly-language-tutor/... ) was the one that finally made it click for me. Zaks's book was an older one -- targeting the KIM-1, IIRC. It didn't have any Apple II-specific content, but it was still interesting to read once I'd worked my way through Haskell's book.

Richard Haskell is still writing accessible introductory books for Verilog and VHDL. His new books look pretty good, but they came along a bit too late to help me this time.