Actually, the documentation was amazing in retrospect. The Sinclair Spectrum user manual, which started with how to connect the computer to the TV, then proceeded to teach BASIC, had, in the appendix at the back, the most fascinating table. IIRC, it was a combined ASCII and Z80 opcode table, from which I pretty much taught myself assembler (and committed to memory the most common opcode hex encodings - and I still remember some of them to this day!).

This! Still have fond memories of reverse-engineering the screen-buffer encoding on my Amstrad CPC 464 by poking values directly into the buffer and watching what happened on the display (i.e. my TV). Somehow got hold of the Z80 datasheet for opcodes - but working out the relative jumps with pen an paper was a slow process. Upgraded to Amiga around the time when I realized that assemblers had to be a thing...

Yes, I was coding in hand-coded hex machine language for a couple of years before I had access to an assembler. These days, when and IDE drops into assembly and all my 20-something colleagues duck for cover, I rub my hands together in glee...