I had a Radio Shack Color Computer and Radio Shack actually sold a ROM cartridge (right alongside all the game cartridges) that was an editor/assembler. That's how I got started in assembler. The manual that came with it covered using the assembler itself but only had a minimal overview of 6809 CPU assembly language. Pretty strange that you could pick up an assembly language IDE at your local Radio Shack store (with over 7000 locations, virtually every town in the U.S. had at least one Radio Shack).
My teenaged self actually looked up Motorola in the phone book and called their offices asking for information on how to program their CPU. Some nice salesperson there took pity on me and sent me their 6809 reference manual along with a quick reference card for free. The manual was quite a sizable book that I wasn't fully ready to understand yet but that reference card was my constant companion. I still have it today. :-)
Those reference cards are very helpful. In college the embedded PC boards (generic) x86 and (I forget if it was a 68 or 65 series CPU) each included a CPU manual and quick assembly reference card. Seeing instructions grouped by how the decoder wanted them, it was much easier to see where a given entry would have 3 or 4 bits utilized to decode a sub function among a general class of instructions.
Not OP, but In 1983 I was 13 and I won a scholarship to a summer camp that rented space in a rich kid prep school for computer camp. I learned pascal over two weeks and the following summer I went back and learned assembly. Three years later I built a 286 and fortunately lived near the Yale bookstore and it had a book on 286 assembly. Basically being a middle class kid adjacent to rich people is how I learned assembly at a young age. If it wasn’t for that camp I wouldn’t have learned computer architecture, logic, and assembly until college. Zip code matters.
My C64 Programmers Reference Guide taught BASIC, and had a memory map of the computer, documentation for all the custom chip registers, all the opcodes for the 6510 CPU, chip timing diagrams, I/O port pinouts, and a full schematic of the C64 in the back of the manual. I'm sure not all C64s came with that, this was the old-school 2-inch-thick C64 manual that came with early computers, at least mine had it. From this manual I learned assembly language in about a week in 1986 while I was still in junior high school. It was pretty amazing. It definitely ignited my passion for programming, and I still code assembly today on embedded MCUs.
There was a third-party Machine Code Monitor cartridge. Don't remember who made it. You could write code, as well as view it, or debug it.
I used the 6502 manual. I was taking Machine Code in school at the time (tech school -not "proper" school), and had learned how to trawl the tech literature for guidance.
3KB of RAM. So little room, I needed to write most of my apps in Machine Code. That was OK. At school, I had an STD Bus-Based 6800, with 256B.
Was a very good learning experience.