For a general introduction to LaTeX, including the new features of LaTeX2e, you should read LaTeX: A Document Preparation System by Leslie Lamport [].
A more detailed description of the new features of LaTeX, and an overview of more than 150 packages, is to be found in The LaTeX Companion by Michel Goossens, Frank Mittelbach and Alexander Samarin [].
The LaTeX system is based on TeX, which is described in The TeXbook by Donald E. Knuth [].
There are a number of documentation files which accompany every copy
of LaTeX. A copy of LaTeX News will come out with each
six-monthly release of LaTeX, and is found in the files
ltnews*.tex. The author's guide LaTeX2e for Authors describes the new
LaTeX document features; it is in usrguide.tex. The guide
LaTeX2e Font Selection describes the LaTeX font selection scheme for
class- and package-writers; it is in fntguide.tex. Configuring
LaTeX is covered by the guide Configuration options for LaTeX2e in
cfgguide.tex whilst the philosophy behind our policy on
modifying LaTeX is described in Modifying LaTeX in
modguide.tex.
We are gradually turning the source code for LaTeX into a LaTeX
document LaTeX: the program. This document includes an index of
LaTeX commands and can be typeset from source2e.tex.
For more information about TeX and LaTeX, please contact your local TeX Users Group, or the international TeX Users Group. Here are some useful addresses.
TeX Users Group, P.O. Box 1239, Three Rivers, CA 93271-1239, USA
Fax: +1 209 561 4584 Email: tug@mail.tug.orgUK TUG, 1 Eymore Close, Selly Oak, Birmingham B29 4LB, UK
Fax: +44 121 476 2159 Email: uktug-enquiries@tex.ac.uk