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The essential TeX/LaTeX Books
By an Amazon.com customer
A Guide to LATEX: Document Preparation for Beginners and Advanced Users (3rd Edition)A Guide to LATEX: Document Preparation for Beginners and Advanced Users (3rd Edition) by Helmut Kopka
Buy used from: $8.37
By far the best introduction to LaTeX
The Latex Companion (Addison-Wesley Series on Tools and Techniques for Computer T)The Latex Companion (Addison-Wesley Series on Tools and Techniques for Computer T) by Michel Goossens
Buy used from: $12.37
It's starting to show its age, but is still invaluable for advance users
TeX Unbound: LaTeX and TeX Strategies for Fonts, Graphics, and MoreTeX Unbound: LaTeX and TeX Strategies for Fonts, Graphics, and More by Alan Hoenig
Buy new: $53.55 / Used from: $39.98
Fantastic resource: everything you always wanted to know about TeX/LaTeX and fonts.
The LaTeX Graphics Companion: Illustrating Documents with TeX and Postscript(R)The LaTeX Graphics Companion: Illustrating Documents with TeX and Postscript(R) by Michel Goossens
Buy used from: $7.48
The LaTeX Web Companion: Integrating TeX, HTML, and XMLThe LaTeX Web Companion: Integrating TeX, HTML, and XML by Michel Goossens
Buy new: $38.99 / Used from: $1.93
xml, pdf and LaTeX: how they can be made to work excellently together
The TeXbookThe TeXbook by Donald E. Knuth
Buy new: $36.51 / Used from: $4.98
From the Man himself. Brillant, but obscure and not all that efficiently organized
Tex by Topic: A Texnician's ReferenceTex by Topic: A Texnician's Reference by Victor Eijkhout
Buy used from: $48.98
Unfortunately out-of-print: A clear and very compact reference book on the inner workings of TeX