While Knuth's book is the definitive reference for TeX, there are other books covering TeX:
The TeXbook by Donald Knuth (Addison-Wesley, 1984, ISBN 0-201-13447-0, paperback ISBN 0-201-13448-9)
A Beginner's Book of TeX by Raymond Seroul and Silvio Levy, (Springer Verlag, 1992, ISBN 0-387-97562-4)
TeX by Example: A Beginner's Guide by Arvind Borde (Academic Press, 1992, ISBN 0-12-117650-9)
Introduction to TeX by Norbert Schwarz (Addison-Wesley, 1989, ISBN 0-201-51141-X)
A Plain TeX Primer by Malcolm Clark (Oxford University Press, 1993, ISBNs 0-198-53724-7 (hardback) and 0-198-53784-0 (paperback))
TeX by Topic by Victor Eijkhout (Addison-Wesley, 1992, ISBN 0-201-56882-9 - now out of print, but see (La)TeX tutorials)
TeX for the Beginner by Wynter Snow (Addison-Wesley, 1992, ISBN 0-201-54799-6)
TeX for the Impatient by Paul W. Abrahams, Karl Berry and Kathryn A. Hargreaves (Addison-Wesley, 1990, ISBN 0-201-51375-7)
TeX in Practice by Stephan von Bechtolsheim (Springer Verlag, 1993, 4 volumes, ISBN 3-540-97296-X for the set, or Vol. 1: ISBN 0-387-97595-0, Vol. 2: ISBN 0-387-97596-9, Vol. 3: ISBN 0-387-97597-7, and Vol. 4: ISBN 0-387-97598-5)
TeX: Starting from Square One by Michael Doob (Springer Verlag, 1993, ISBN 3-540-56441-1)
The Advanced TeXbook by David Salomon (Springer Verlag, 1995, ISBN 0-387-94556-3)
A collection of Knuth's publications about typography has recently been published:
Digital Typography by Donald Knuth (CSLI and Cambridge University Press, 1999, ISBN 1-57586-011-2, paperback ISBN 1-57586-010-4).
and in late 2000, a “Millennium Boxed Set” of all 5 volumes of Knuth's “Computers and Typesetting” series (about TeX and Metafont) was published by Addison Wesley:
Computers & Typesetting, Volumes A-E Boxed Set by Donald Knuth (Addison-Wesley, 2001, ISBN 0-201-73416-8).
For LaTeX, see:
LaTeX, a Document Preparation System by Leslie Lamport (second edition, Addison Wesley, 1994, ISBN 0-201-52983-1)
A guide to LaTeX2e Helmut Kopka and Patrick W. Daly (third edition, Addison-Wesley, 1998, ISBN 0-201-39825-7)
The LaTeX Companion by Michel Goossens, Frank Mittelbach, and Alexander Samarin (Addison-Wesley, 1993, ISBN 0-201-54199-8)
The LaTeX Graphics Companion: Illustrating documents with TeX and PostScript by Michel Goossens, Sebastian Rahtz and Frank Mittelbach (Addison-Wesley, 1997, ISBN 0-201-85469-4)
The LaTeX Web Companion: Integrating TeX,
HTML and XML by Michel Goossens and Sebastian Rahtz (Addison-Wesley, 1999, ISBN 0-201-43311-7)
TeX Unbound: LaTeX and TeX strategies for fonts, graphics, and more by Alan Hoenig (Oxford University Press, 1998, ISBN 0-19-509685-1 hardback, ISBN 0-19-509686-X paperback)
Math into TeX: A Simplified Introduction using AMSLaTeX by George Grätzer (Birkhäuser, 1993, ISBN 0-817-63637-4, or, in Germany, ISBN 3-764-33637-4)
Math into LaTeX: An Introduction to LaTeX and AMSLaTeX by George Grätzer (third edition Birkhäuser and Springer Verlag, 2000, ISBN 0-8176-4431-9, ISBN 3-7643-4131-9)
A list of errata for the first printing is available from: http://www.springer-ny.com/catalog/np/jan99np/0-387-98708-8.html
First Steps in LaTeX by George Grätzer (Birkhäuser, 1999, ISBN 0-8176-4132-7)
LaTeX: Line by Line: Tips and Techniques for Document Processing by Antoni Diller (second edition, John Wiley & Sons, 1999, ISBN 0-471-97918-X)
LaTeX for Linux: A Vade Mecum by Bernice Sacks Lipkin (Springer-Verlag, 1999, ISBN 0-387-98708-8, second printing)
A sample of George Grätzer's book, in Adobe Acrobat format, is also available ftp://cam.ctan.org/TeX-archive/info/mil/mil.pdf
Example files for the LaTeX Graphics and Web Companions are available in ftp://cam.ctan.org/TeX-archive/info/lgc.tar.gz (Graphics) and ftp://cam.ctan.org/TeX-archive/info/lwc.tar.gz (Web).
Example files for George Grätzer's 'First Steps' are available in ftp://cam.ctan.org/TeX-archive/info/FirstSteps.tar.gz
The Metafontbook by Donald Knuth (Addison Wesley, 1986, ISBN 0-201-13445-4, ISBN 0-201-52983-1 paperback)
Alan Hoenig's 'TeX Unbound' includes some discussion and examples of using Metafont.
see also: LaTeX and LyX