The Internet Archive has tons of old scanned books on the subject. Computers were new to a LOT of people at the time most of these were published, so they usually assume minimal knowledge.
Personally, I grew up on COMPUTE! magazine, and found their books to be very good when I was a kid learning this stuff.
There's a nice 6502 assembly intro + Sim here :
https://skilldrick.github.io/easy6502/
Added edit: I mean, it's not PET-specific, but it's cool to have a little sim with a little chunk of display memory to play with.
Also you'll quickly find that assembly is extremely verbose. Learn how to load registers and jump to (and return from) subroutines as quick as you can to prevent endless amounts of repetitive assembly.
If you have little to no experience with programming at all, I suggest trying the assembly game TIS-100 first to wrap your head around the concepts, rather than trying to learn 6502 right away.
I would pair it with this tutorial, which is really just a bottom-up programming course.
Python is very different from assembly, but assembly isn't hard to learn if you apply yourself and take it slow. It will seem kind of abstract without something like the game above to challenge your understanding.
This project is mostly about building a 6502 but the later videos are more about programming it
This guy has made a whole lot of tutorials for various assembly languages and systems, including the pet. The website has text tutorials, but if you prefer he also has a YouTube channel.
Wow, what a place to start!
It's a bit off the mark, but I'll mention the Nybbles and Bytes youtube videos because I think they're pretty neat.
https://www.youtube.com/playlist?list=PLKrMlwGUrEZhdcJX5gAYAsBFGl-nLvmMM
I remember reading thru Apple II Assembly by Hyde or something. It's been a while, and I didn't have the brain power back then (now I do, but not the time). I remember it got some funky illustration making the book quite a fun one to go thru.
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