A free reference manual for LaTeX2e in the CTAN is written in GNU Texinfo, and available in Info, PDF, and HTML:

There’s also another free user manual for LaTeX, started six years ago, in 1999. It is far from completion, but has some interesting commentary and a great command reference appendix. It is written in GNU Texinfo and is availble as a helpful Emacs InfoMode. PDF versions are also available:

The documentation’s project page, that has been stale since 2002, is here:

There’s a more complete free manual on LaTeX available from the India TeX Users Group (TUG). Unfortunately, it was written in LaTeX of all things: