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!).