I was lucky. I was able to afford the Model I Level II with 16K of RAM using my baby-sitting money!
And Radio Shack had more than just the BASIC programming manual. They also had a Technical Reference manual that explained the full theory of operation including the timing, the generation of the video signal, the reading of the character data out of ROM...Everything!
I learned enough about digital electronics from that single manual to design my own programmable EPROM board that allowed me to rewrite the character font table.
It also helped me repair the unit after a mouse crawled inside and urinated all over the main board dissolving a number of traces.
The ATARI 800 I purchased a few years later had great documentation, too. The entire kernel was explained. MAN! What happened? Nothing comes with documentation any more!