There actually was an amazing little book published by Compute! that contained information on every memory address of interest on the Atari 400/800, and what peeking/poking different values at each address did. It was fantastic, and a tinkering teenager's bible.
Amusingly, I have a floppy on my desk that my little brother wrote on when he was about 4 or 5. It has two commands on the label - POKE and PEAK :)