The TeXbook by Donald Knuth, ISBN 0201134489
The TeXbook is DonKnuth's authoritative guide on how to use Tex. It's [TeX or the book?] really really hard, but if you use TeX, it's really really worth the effort. It's much more useful than Leslie Lamport's LaTeXbook, which I found to be very poorly organised simply to support a pun in Appendix C.
I agree. I use PlainTex? rather than LaTeX precisely because the documentation's so much better. -- JasonGrossman
I use Plain TeX rather than LaTeX because Plain TeX works better and I can understand it, I find it more versatile and useful and less bloated than LaTeX. I have neither book, but I have tried using LaTeX once and I didn't like it. Plain TeX is far superior!
See LiterateProgramming, TexLanguage