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.
First you have to define a layout with enough margin space.
paperwidth=170mm, paperheight=240mm, left=142pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt
The asymmetric layout is a little bit tricky. If I understand correctly, you want the margin and text width to be the same, but the page is shifted towards the outer edge. This can be achieved with a binding offset.
bindingoffset=30pt,asymmetric
the latter option makes the margin always appear on the same side. Now, you have to switch the \marginpars to appear at the inner side, this is done with the
reversemarginpar
option. I also used showframe to make everything more obvious. In total:
\documentclass{book}
\usepackage[paperwidth=170mm, paperheight=240mm, left=142pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt,bindingoffset=30pt,showframe,asymmetric,reversemarginpar]{geometry}
\usepackage{lipsum}
\begin{document}
~\newpage
\marginpar{\lipsum[3]}\lipsum[2]
\clearpage
\marginpar{\lipsum[3]}\lipsum[4]
\end{document}
which gives you
It is not clear, whether you want tufte-latex or not. Please provide an MWE and I will try to edit the answer.
EDIT:
If you are willing to try something new, you might like sidenotes. It is a package I made to add the T-L functionality to normal LaTeX classes. You can get the newest version on github. So in principle, a question such as 'How to do that in T-L' becomes 'How to do that in LaTeX'. This should give you a lot of options. In your particular case:
\documentclass{book}
\usepackage{sidenotes}
\usepackage{graphicx}
\usepackage[paperwidth=170mm, paperheight=240mm, left=142pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt,bindingoffset=30pt,asymmetric,reversemarginpar]{geometry}
\usepackage{lipsum}
\begin{document}
~\newpage
\lipsum[1]
\begin{marginfigure}
\includegraphics[width=100pt]{rectangle}
\caption{I am a figure}
\end{marginfigure}
\lipsum[2]
\clearpage
\begin{margintable}%
\small
\begin{tabular}{lll}
Hg&Sn&Pb \\
0.50&0.47&0.48\\
\end{tabular}
\caption{I am a table.}%
\end{margintable}%
\lipsum[4]
\end{document}
which gives you:
I do not know the answer to your second question off-hand, but \begin{adjustwidth} should be a good starting point. With the sidenotes package you might be able to look for a generic LaTeX answer and use that.
Best Answer
I would say that the book class (nor any others) does not saves space for margin notes; rather, it allows you to specify the maximum space that you want to allow for marginalia so that they do not exceed the specified margin widths.
In two sided printing (e.g., for books) the basic layout is a double spread --- left hand and right hand pages open where the width of the outer margins are equal and the apparent width of the gutter (the distance between the left hand and right hand textblocks) is the same, i.e, all three white columns are the same width.
See any good book on typography; Robert Bringhurst's `The Elements of Typographic Style' is one of the best.