Standard Document Classes for LaTeX version 2e - Annotated code from the standard document classes. Useful for those who wish customize pre-defined commands and environments. - http://tex.loria.fr/ctan-doc/macros/latex/base/classes.pdf
Wikibooks: LaTeX - A collaboratively written guide to the LaTeX markup language. - http://en.wikibooks.org/wiki/LaTeX
The Comprehensive LaTeX Symbol List - Organized list of over 2500 symbols commonly available to LaTeX users (either by default or by including special packages). Available in ps and pdf formats. - http://www.ctan.org/tex-archive/info/symbols/comprehensive/
Modifying LaTeX - Describes the principles and policy on the distribution and modification of the LaTeX system. - http://www.ctan.org/tex-archive/macros/latex/doc/modguide.pdf
LaTeX2e for Class and Package Writers - An introduction to writing classes and packages for LaTeX2e. Includes information about updating existing LaTeX 2.09 style files. - http://www.ctan.org/tex-archive/macros/latex/doc/clsguide.pdf
LaTeX2e for Authors - Describes how to take advantage of the new features in LaTeX2e, and also how to process documents from older versions. - http://www.ctan.org/tex-archive/macros/latex/doc/usrguide.pdf
Configuration options for LaTeX2e - Describes the possibilities for configuration of this release of LaTeX, including how to configure font definition files to take advantage of available fonts. - http://www.ctan.org/tex-archive/macros/latex/doc/cfgguide.pdf
LaTeX2e Font Selection - Describes the new font selection features of LaTeX2e. Aimed at those writing font-loading packages similar to times or latexsym. - http://www.ctan.org/tex-archive/macros/latex/doc/fntguide.pdf
LaTeXe Help 1.6 - Documentation on LaTeX written by Torsten Martinsen. - http://www.ctan.org/tex-archive/info/latex2e-help-texinfo/latex2e.html
LaTeX Help - On-line manual at Emory University. - http://www.emerson.emory.edu/services/latex/latex_toc.html