To be fair, writing a book about Emacs is a Sisyphean effort by definition - this sea is a bottomless abyss of hackery abundance, and any meaningful effort to explain it is worth a celebration.
On Elisp and multithreading/processing, well, just look at bordeaux-threads in Common Lisp where the support is not universal for Clisp. SBCL and ECL work, but...
You are for emacs, happy to see you here... :)
I was having trouble accessing the Digitala Vetenskapliga Arkivet site (linked in headline) directly, so uploaded it here (link expires in 3 days):
I started the Hacker's guide on the emacs wiki many years back. I think this doc was much needed.