Edit: Which is number 2 on this list hah.

I have thought about writing something around type checking. That aspect of compilers feels a little underserved to me. Types and Programming Languages does a brilliant job of talking about designing and understanding type systems. But it's less focused on just sitting down and building a working type checker. Most other compiler books seems to gloss over it quickly so they can rush to the fun back end stuff.

But it's hard because the way you implement the type checker depends so heavily on the underlying type system and those vary widely. Does the type system have subtyping or not? Are generics monomorphized? Etc. So I have no plans to write anything about that any time soon.

Also, between my first two books, I wrote almost every day for about a decade, so taking a nice long break from that has been nice.