Silvio Levy & Raymond Seroul 
Beginner’s Book of TEX [PDF ebook] 

The last two decades have witnessed a revolution in the realm of typography, with the virtual disappearance of hot-lead typesetting in favor of the so-called digital typesetting. The principle behind the new technology is simple: imagine a very fine mesh superimposed on a sheet of paper. Digital typesetting consists in darkening the appropriate pixels (tiny squares) of this mesh, in patterns corresponding to each character and symbol of the text being set. The actual darkening is done by some printing device, say a laser printer or phototypesetter, which must be told exactly where the ink should go. Since the mesh is very fine-the dashes surrounding this sentence are some six pixels thick, and more than 200 pixels long-the printer can only be controlled by a computer program, which takes a "high-level" description of the page in terms of text, fonts, and formatting commands, and digests all of that into "low-level" commands for the printer. TEX is such a program, created by Donald E. Knuth, a computer scientist at Stanford University.