I'm excited to hear that @mraleph is going to write a book on the dark art of VM implementation!
https://evolvingavm.com/
Related Posts
On thinking of books like software, treating them as evolving artifacts, and URLs versus paper book publishers: https://cs.brown.edu/~sk/Memos/Books-as-Software/
Over a sufficiently long time horizon, all code you write is legacy code.
An ad hoc, informally-specified, bug-ridden, slow implementation of half of a text adventure game -- or as I like to call it, a compiler.