> Today certainly one cannot expect portability with all of the commands
> described in the Companion.

No, that was never the intention of the companion.
But conversely The core latex book would not work for this either.
You are going to need the AMS math stuff, probably the tabular
extensions from latex/tools and then some of the contributed packages
like natbib and cite. The exact list would need some care to be drawn up
but a starting point should probably the core set of packages in use
now. Rather than what was available in 1992/3 when those books were
being drawn up.

David