I don't know if there's a more complete online manual for LaTeX, but I
find "Hypertext Help with LaTeX" which can be found at any of these
mirror sites
http://wwwsyseng.anu.edu.au/~sito/latex/mirror.html
to be quite useful for quick reference. Of course it's faster to access
if you download the tarred version and install it on your system.
However, for serious work I cannot live without my "LaTeX Companion".