The absolutely standard place to start is "LaTeX: A Document Preparation System" by Leslie Lamport, the original author of the LaTeX macros on top of TeX.
When I started my PhD, I asked if I could borrow my supervisors copy. He said yes, but that I should get my own. He was right.
It is extremely accessible and well written. Given the higher technical nature, it is quite compact, and seriously manages to say exactly what you need, in a form that allows you to find it easily.
I've tried to use online books etc when I've been without my copy. They aren't even nearly as useful.