Computers & Typesetting, Volume A: The TeXbook

Here is the definitive guide to the use of TEX, written by the system's creator, Donald E. Knuth.
TEX represents the state-of-the-art in computer typesetting. It is particularly valuable where the document, article, or book to be produced contains a lot of mathematics, and where the user is concerned about typographic quality. TEX software offers both writers and publishers the opportunity to produce technical text of all kinds, in an attractive form, with the speed and efficiency of a computer system.