If people are wondering why the next chapter of my NixOS in Production book is taking so long it's because I'm working on this pull request to add macOS support for NixOS tests (which is pretty instrumental for the chapter I'm in the process of writing). Thankfully, that work seems like it's almost done.
More generally, a lot of what you're paying for when you buy my book is not just the book itself but all of the supporting open source work I do to streamline the development experience outlined in the book (including stuff like terraform-nixos-ng and nixpkgs#darwin.linux-builder).