Reference documents
For TeX primitive commands a rather nice
quick reference booklet,
by John W. Shipman, is available; it’s arranged in the same way as the
TeXbook. By contrast, you can view David Bausum’s
list of TeX primitives
alphabetically or arranged by “family”. Either way, the list has a
link for each control sequence, that leads you to a detailed
description, which includes page references to the TeXbook.
There doesn’t seem to be a reference that takes in Plain TeX as
well as the primitive commands.
Similarly, there’s no completely reliable command-organised reference
to LaTeX, but the NASA
Hypertext Help with LaTeX is
recently much improved. It still talks in LaTeX 2.09-isms in places,
but it’s been updated for current LaTeX; there are a number of
mirrors of the site, and it may be worth choosing a “local” one if
you’re going to use it a lot.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ref-doc