The one that was the go-to manual back in the days was the Atari Compendium. I still have the book on my shelf.
I have seen digital versions (scans, I guess) floating around online. I don't know how complete they are.
There is still a demoscene for the Atari, although it's of course much smaller these days. You can find some pointers on our group's website: www.dhs.nu