Online introductions: Plain TeX
Michael Doob’s splendid ‘Gentle Introduction’ to Plain TeX
(available on CTAN) has been stable for a very long time.
Another recommendable document is D. R. Wilkins ‘Getting started with TeX’,
available on the web at http://www.ntg.nl/doc/wilkins/pllong.pdf
- Gentle Introduction
- info/gentle/gentle.pdf
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=man-tex