Ask HN: Favourite resources for learning programming type theory?

Hello HN,

Recently, I've been massively enjoying diving into type checkers and how to effectively use them (mostly in Python).

I lack foundational knowledge about the topic (I never studied CS), however, so I am looking for introductory textbooks/blogs/resources on the topic.

More specifically: I see terms being thrown around like "Algebraic sum type", "covariant", "contravariant", and I would like to understand more about what they mean and imply in a practical sense.

Thanks!