As others have said, you can use the tufte-book document class. It offers captions in the sidenotes area and a fullwidth environment that stretches across the main text block and the sidenotes area.
\documentclass{tufte-book}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{figure}[!ht]
\centering
\rule{4cm}{2cm}
\caption{A black rectangle with lots of text so it fills up some space.}
\end{figure}
\lipsum[1]
\begin{table}[!ht]
\centering
\rule{4cm}{2cm}
\caption{A black rectangle with lots of text so it fills up some space.}
\end{table}
\begin{fullwidth}
\lipsum[1]
\end{fullwidth}
\end{document}
If you don't want to use the tufte-book class, then you can use one of the standard ones (book, report) together with the geometry package to change the page layout, the floatrow package to have captions in the margins, and the changepage package to change the margins for a part of a document:
\documentclass{book}
\usepackage[margin=4cm,marginparwidth=3cm]{geometry}
\usepackage{floatrow}
\usepackage{changepage}
\usepackage{lipsum}
\floatsetup{margins=hangleft,capposition=beside,
capbesideposition={top,left},floatwidth=\textwidth}
\begin{document}
\lipsum[1]
\begin{figure}[!ht]
\centering
\rule{4cm}{2cm}
\caption{A black rectangle with lots of text so it fills up some space.}
\end{figure}
\lipsum[1]
\begin{table}[!ht]
\centering
\rule{4cm}{2cm}
\caption{A black rectangle with lots of text so it fills up some space.}
\end{table}
\begin{adjustwidth}{-3cm}{}
\lipsum[1]
\end{adjustwidth}
\end{document}
You could also consider using one of the classes from the KOMA-Script bundle (scrbook, scrreprt) or the memoir document class and adapting them to your needs.
Best Answer
From http://www.latex-community.org/forum/viewtopic.php?f=5&t=2712
Using
\the\textwidthwill allow you to see the value LaTeX is using. However, the default unit of measurement is probably points, not a likely option for your graphics software.Fortunately, the conversion function is available in the standard
layoutspackage:\usepackage{layouts}...textwidth in cm: \printinunitsof{cm}\prntlen{\textwidth}will print the value in cm. All the usual LaTeX units of measurement are also available if cm aren't what you want.