Ahh, did just reply to your question on Atari Forum but realised you posted here too! Further to what I said there, I sorted through and sent Chris PDF's of the Compute! mag's tutorials for any STOS compatible listings here:
https://www.exxoshost.co.uk/atari/STOS/ ... 0pages.zip
And what I posted at AF:
For STOS Basic, go here:
https://www.exxoshost.co.uk/atari/STOS_index.htm
There's games with source code there, a documentation section now too with PDF's, etc. Also on that site, scroll down to "S" within the Extensions & Source section and you will find STOS Buddy, which again has more source code for games, plus lots of disk mag articles.
Depending on how you allocate your time, you will possibly find months worth of reading and exploring there.