LaTeX Quick Reference

Documentation

An excellent introdutory book available online is The Not So Short Introduction to LaTeX 2e (the file lshort.pdf).
The AMS-LaTeX homepage has documentation for the various AMS packages, with the most important being the amsmath documentation.
Gratzer's books Math into Latex and More Math into Latex are also quite useful. The tables and reference parts of Math into Latex are available for download here.
Ghostscript can do many complex things to PS and PDF files. Documentation is available at the ghostscript homepage.
NIST has a good summary of the rules for typesetting quantities with units, as well as a summary of the use of typefaces for mathematical symbols.

To produce bold italic math symbols use \boldsymbol from the amsmath package.
Dimensionless quantities like the Reynolds number and Knudsen number can be typeset with \mathit{Re} and \mathit{Kn} as they are single words, not the separate variables R and e or K and n.

To join/concatenate two PDF files into a single file, use the ghostscript command

gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=joined.pdf file1.pdf file2.pdf

The same technique will also work for PS files, but with -sDEVICE=pswrite.