www.ctan.org/tex-archive/info/lshort/english/lshort.pdf

www.maths.tcd.ie/~dwilkins/LaTeXPrimer/

www.emerson.emory.edu/services/latex/latex_toc.html

Also, I highly recommend the book, Learning Latex, by David Griffiths and Desmond Higham, published by SIAM.