A getting started guide for latex and a more in depth tutorial. It also has a sandbox where you can try out latex math commands. The ads are pretty obstructive though.
This is a great reference to keep handy when working on a latex document. It covers most common areas of LaTeX in great detail, and offers good extension packages to use for special formatting cases. It also allows for searching.
LaTeX is commonly used for writing dissertation and theses. The following researcus contain LaTeX templates and classes specifically designed for the theses requirements at Penn State.