Beginner - Setting Up LaTeX
Are you a total beginner to LaTeX? I was once in that position and I remember being completely bewildered as to how to even get started. Setting Up LaTeX is a one-page set of instructions for what to download and install to get started using TeXnicCenter, one of the most popular LaTeX interfaces (and what I use).
Beginner/Intermediate - Useful LaTeX
If you're like me, you don't need to be writing journal-quality documents, but you want to be able to write up homework and handouts in LaTeX when you feel like it. Useful LaTeX is a quick intro, plus a collection of useful and common commands that I've built up over my travels across the web, in 27 pages. I decided to teach myself LaTeX one summer and made this manual as a project--but even now, years later, I pull it up frequently when I'm writing things up. Truly useful in my experience!
Advanced - The Not So Short Introduction to LATEX2E
If, however, you are looking for a very thorough list of commands and how to use them, I recommend The Not So Short Introduction to LATEX2E by Tobias Oetiker, Hubert Partl, Irene Hyna, and Elisabeth Schlegl. I wouldn't recommend diving right into this if you have no prior experience with LaTeX, as it can be a bit overwhelming at 139 pages. I have included some of the material most relevant to the average LaTeX user in Useful LaTeX, so many thanks to them!
Also useful is The Comprehensive LaTeX Symbol List by Scott Pakin, a truly comprehensive collection of 2590 symbols and their corresponding commands. Again, I have included some of the material most relevant to the average LaTeX user in Useful LaTeX, so many thanks to Scott!