Not only was the references supplied by Apple good, I think it was also the beginning of the advanced "hacker's" manuals available from 3rd parties. I remember a book titled "Inside the Apple][." It documented every memory call in the ROM and the basic interperator. (Remember PEEKing and POKEing?)
Beagle Bros. software also had a collection of Hack tools and subroutines that could be freely (If I remember correctly) incorperated into user programs. All fully documented. I remember trapping Ctrl-C inputs with ONERR and redirecting CTRL-OPENAPPLE-RESET back to the beginning of program execution. FUN STUFF!
I know one thing that has changed. Tight, efficient code was a must on that machine, unlike the bloatware we see today 30 years later.