People have long argued for (La)TeX books to be made available on the web, and until relatively recently this demand went un-answered.
The first to appear was Victor Eijkhout’s excellent “TeX by Topic” in 2001 (it had been published by Addison-Wesley, but was long out of print). The book is now available on CTAN; it’s not a beginner’s tutorial but it’s a fine reference. It’s also available, as a printed copy, via the on-line publishers Lulu (not quite free, of course, but not a bad deal…).
Addison-Wesley have also released the copyright of “TeX for the Impatient” by Paul W. Abrahams, Karl Berry and Kathryn A. Hargreaves, another book whose unavailability many have lamented. The authors have re-released the book under the GNU Free Documentation Licence, and it is available from CTAN.
Norm Walsh’s “Making TeX Work” (originally published by O’Reilly) is also available (free) on the Web, at http://makingtexwork.sourceforge.net/mtw/; the sources of the Web page are on CTAN. The book was an excellent resource in its day, but while it is now somewhat dated, it still has its uses, and is a welcome addition to the list of on-line resources. A project to update it is believed to be under way.
This answer last edited: 2011-06-29