If there's no way to regenerate book.html it should be removed and redirect to https://docs.freebsd.org/en/books/porters-handbook/
jrm@ also managed to find two more outdated versions
There's a newer copy of it here in pdf format (jrm@ provided a link)
We should clean that.
There is a single/full version of the porters-handbook in this link:
It's still not listed anywhere.