I'd be curious if there's anything people would consider their primary source, I find myself picking concepts from various sources when the need arises.

Here's are a couple from poking around on the topic:

https://css-tricks.com/intrinsic-typography-is-the-future-of...

https://github.com/clagnut/webtypography

https://cssreference.io/typography/

https://type-scale.com/