TeXbook by Donald Knuth

This guide to computer typesetting using TeX is written by Donald Knuth, the system's creator. TeX offers both writers and publishers the ability to produce plain or technical text, especially text containing a great deal of mathematics, comparable to the work of the finest printers. Novice and expert alike will find The TeXbook useful; it is accessible to the beginner, and also contains the details required by experienced users.

From the uploader: Mr. Knuth, sorry for the copyright infringement, but if you say about "open-TeX approach", then we cannot understand why you are such an opponent to the distribution of your core TeX book. There is no realistic way in some countries to get it.