My parents bought me one of those new VIC-20 computing machines back when they were newly released.
Having never really been exposed to BASIC programming, except for many long sessions in front of TRS-80's, much to the chagrin of Radio Shack managers, I found the VIC-20 user manual rather... "light"... in its content.
Later, I bought the Programmer's Reference Guide, which was an explosion of useful details and information about the VIC-20!!! There was an entire section on memory maps and detailed 6502 assembler codes. Using just that book, I started learning to write and hand compile machine language routines. I would enter the programs via PEEK commands and DATA statements.