Crafting Interpreters was a great resource for parsing, but working on the static type system for my language project I'm mostly feeling my way around in the dark and making stuff up as I go. That's going better than expected, but I still feel I would benefit from something at around the same level as CI but for type systems (checking and inference; mostly inference, but also things like generic types and refinement). Something practical and light on formal theory, with insight from industry, that lays a foundation from which to keep learning. I tried reading about Hindley-Milner etc directly, and it went way over my head.