The Not So Short Guide To LaTeX has always been a godsend to me, and it's free.

I loved More Math Into LaTeX. Don't worry about the "more" in the title, that just represents a new edition. I used it to learn LaTeX from scratch.

http://www.amazon.com/More-Math-Into-LaTeX-Edition/dp/038732...

"The LaTeX Companion" proved very useful when I wrote my thesis. [http://www.amazon.com/LaTeX-Companion-Techniques-Computer-Ty...]

One of my professors has a number of video tutorials which can be found here: http://castingoutnines.wordpress.com/2010/02/04/12-videos-fo...

There is a very reasonable wikibook: https://en.wikibooks.org/wiki/LaTeX