I've been implementing a language with a TypeScript-like type system, and while the core system is pretty intuitive (values combining with other values, determining whether basic value types fit into others), some of the fringes have been challenging:

- Generics, especially inferring generic type params

- Recursive types

- Deeply mutable vs immutable types

- Type refinement based on checks/usage

- Giving good error messages for deep type mismatches (though I'm not sure TypeScript itself has figured this one out yet, lol)

etc. And even the stuff I've figured out, I figured out almost all from scratch (sometimes having to bail and take a totally new approach midway through). I would love to read a book giving me a proper bottom-up walkthrough of the current landscape of type system implementation