[texhax] textwidth question take two

[Robert E. Harvey, M.D.]

I'm having some problems with the syntax of   
\setlength{\texdtwidth}{lengthcm} as documented in "A Guide to Latex", 
third edition by Kopka and Daly.

If I enter
\documentclass[12pt]{book}
\begin{document}
\noindent
\hoffset=-.5in
\large
\setlength\textwidth=+18cm

I get this error on compiling:

This is TeX, Version 3.14159 (Web2C 7.3.1)
(bomlesson1.tex
LaTeX2e <2001/06/01>
Babel <v3.7h> and hyphenation patterns for american, french, german, 
ngerman, i
talian, nohyphenation, loaded.
(/usr/share/texmf/tex/latex/base/book.cls
Document Class: book 2001/04/21 v1.4e Standard LaTeX document class
(/usr/share/texmf/tex/latex/base/bk12.clo)) (bomlesson1.aux)
! Missing number, treated as zero.
<to be read again>
                  \relax
l.6 \setlength\textwidth=
                        +18cm
?
If I enter
\documentclass[12pt]{book}
\begin{document}
\noindent
\hoffset=-.5in
\large
\setlength{\textwidth}{18cm}

I get no error messages on compiling but the text is not extended to the 
desired length.

Changing \setlength{\textwidth}{18cm} to \setlength{\textwidth}{6 in} 
gives me the same results.  What is the correct literal way to define 
textwidth?

Thanks.

Bob Harvey