"The only criteria I have for good code these days is "Does it work", "Can I understand it"."

Which of course, are not trivial to achieve by any means.

Maybe also add "Can my coworkers understand it?" Which might be a very different metric.

"I would have read, in their place, books on type theory and OS design, Posix, in other words things that actually help me build stuff and get things done and that I subsequently had to read later."

If you want to get very far with any of those, you will need to write "good code", as you have defined it. So maybe a better way to understand good code would be to read the code of large projects implementing the things you want to learn about?