Captions in the margin of a two-sided book, including wide (margin) images.

mycaption.sty

%!TEX root=test.tex

%%
%% This is file `mycaption.sty',
%% generated with the docstrip utility + positioning modifications.
%%
%% The original source files were:
%%
%% mcaption.dtx  (with options: `package')
%% 
%% This is a generated file.
%% 
%% Copyright (C) 2005 by Stephan Hennig <[email protected]>
%% 
%% This file may be distributed and/or modified under the conditions of
%% the LaTeX Project Public License, either version 1.2 of this license
%% or (at your option) any later version.  The latest version of this
%% license is in:
%% 
%%    http://www.latex-project.org/lppl.txt
%% 
%% and version 1.2 or later is part of all distributions of LaTeX version
%% 1999/12/01 or later.
%% 

%http://tex.stackexchange.com/questions/16328/test-if-text-fits-on-one-line

\ProvidesPackage{mycaption}
  [2009/03/13 v3.0 Put captions into the outer document margin (SH)]
\NeedsTeXFormat{LaTeX2e}[1999/12/01]
\RequirePackage[strict]{changepage}
\RequirePackage{etoolbox}
\newcommand*{\mcaption@alignv}{}
\DeclareOption{top}{\renewcommand*{\mcaption@alignv}{t}}
\DeclareOption{bottom}{\renewcommand*{\mcaption@alignv}{b}}
\ExecuteOptions{bottom}
\DeclareOption{v2.2}{%
  \AtEndOfPackage{%
    \let\margincap\mcaption@mcIIdotII%
    \let\endmargincap\endmcaption@mcIIdotII%
    \newcommand*{\margincapalign}{\mcaption@alignv}
  }%
}%
\ProcessOptions\relax
\newlength{\margincapsep}
\newlength{\margincapwidth}
\AtBeginDocument{%
  \setlength{\margincapsep}{\marginparsep}%
  \setlength{\margincapwidth}{\marginparsep}%
  \addtolength{\margincapwidth}{\marginparwidth}%
}
\newcommand*{\mcaption@CaptionLong}{}
\newcommand*{\mcaption@CaptionShort}{}
\newcommand*{\mcaption@CaptionFlag}{}
\newcommand*{\mcaption@Label}{}
\newcommand*{\mcaption@LabelFlag}{}
\newsavebox{\mcaption@ObjectBox}
\newsavebox{\mcaption@CaptionBox}
\newenvironment{mcaption@mcIIdotII}[2][\mcaption@DefaultOpt]{%
  \def\mcaption@DefaultOpt{#2}%
  \def\mcaption@CaptionShort{#1}%
  \def\mcaption@CaptionLong{#2}%
  \gdef\mcaption@CaptionFlag{t}%
  \gdef\mcaption@CaptionOpt{n}%
  \gdef\mcaption@LabelFlag{f}%
  \begin{lrbox}{\mcaption@ObjectBox}%
    \begin{minipage}{\linewidth}%
}{%
    \end{minipage}%
  \end{lrbox}%
  \mcaption@align@boxes%
  \mcaption@output@boxes%
}%
\newenvironment{mcaption@mcIIIdot}[1][n]{%
  \newcommand\ifopt{\expandafter\ifstrequal\expandafter}%
  \ifopt{#1}{wt}{%
    \gdef\mcaption@CaptionWide{t}%
    \gdef\mcaption@CaptionAlign{b}}{}%
  \ifopt{#1}{wb}{%
    \gdef\mcaption@CaptionWide{t}%
    \gdef\mcaption@CaptionAlign{t}}{}%
  \ifopt{#1}{w}{%
    \gdef\mcaption@CaptionWide{t}%
    \if\mcaption@alignv{t}%
      \gdef\mcaption@CaptionAlign{b}%
    \else%
      \gdef\mcaption@CaptionAlign{t}%
    \fi}{}%
  \ifopt{#1}{b}{%
    \gdef\mcaption@CaptionWide{f}%
    \gdef\mcaption@CaptionAlign{b}}{}%
  \ifopt{#1}{t}{%
    \gdef\mcaption@CaptionWide{f}%
    \gdef\mcaption@CaptionAlign{t}}{}%
  \ifopt{#1}{n}{%
    \gdef\mcaption@CaptionWide{f}%
    \gdef\mcaption@CaptionAlign{\mcaption@alignv}}{}%
  \let\mcaption@origcaption\caption%
  \let\caption\mcaption@caption%
  \gdef\mcaption@CaptionFlag{f}%
  \let\mcaption@origlabel\label%
  \let\label\mcaption@label%
  \gdef\mcaption@LabelFlag{f}%
  \begin{lrbox}{\mcaption@ObjectBox}%
    \begin{minipage}{\linewidth}%
}{%
    \end{minipage}%
  \end{lrbox}%
  \let\caption\mcaption@origcaption%
  \let\label\mcaption@origlabel%
  \mcaption@align@boxes%
  \mcaption@output@boxes%
}%
\newenvironment{margincap}{}{}%
\let\margincap\mcaption@mcIIIdot%
\let\endmargincap\endmcaption@mcIIIdot%
\newcommand*{\mcaption@origcaption}{}
\newcommand*{\mcaption@origlabel}{}
\newcommand*{\mcaption@DefaultOpt}{}
\newcommand*{\mcaption@caption}[2][\mcaption@DefaultOpt]{%
  \gdef\mcaption@DefaultOpt{#2}%
  \gdef\mcaption@CaptionShort{#1}%
  \gdef\mcaption@CaptionLong{#2}%
  \gdef\mcaption@CaptionFlag{t}%
  \ignorespaces
}%
\newcommand*{\mcaption@label}[1]{%
  \gdef\mcaption@Label{#1}%
  \gdef\mcaption@LabelFlag{t}%
  \ignorespaces
}%
\newcommand*{\mcaption@align@boxes}{%
  \begin{lrbox}{\mcaption@CaptionBox}%
    \setlength{\@tempdima}{\marginparwidth}%
    \addtolength{\@tempdima}{\marginparsep}%
    \addtolength{\@tempdima}{-\margincapsep}%
    \setlength{\@tempdimb}{\totalheightof{\usebox{\mcaption@ObjectBox}}}%
    \begin{minipage}{\@tempdima}%
      \if\mcaption@CaptionWide t%
        \if\mcaption@CaptionAlign t%
          \vspace{\marginparsep}\vspace{\@tempdimb}%
        \fi%
      \fi%
      \if\mcaption@CaptionFlag t%
        \setlength{\abovecaptionskip}{0pt}%
        \setlength{\belowcaptionskip}{0pt}%
        \caption[\mcaption@CaptionShort]{\strut\mcaption@CaptionLong\strut}%
      \fi%
      \if\mcaption@LabelFlag t%
        \label{\mcaption@Label}%
      \fi%
      \if\mcaption@CaptionWide t%
        \if\mcaption@CaptionAlign b%
          \vspace{\marginparsep}\vspace{\@tempdimb}%
        \fi%
      \fi%
    \end{minipage}%
  \end{lrbox}%
\sbox{\mcaption@ObjectBox}{%
  \if\mcaption@CaptionAlign t\vtop
  \else\vbox
  \fi
  {%
    \vskip0pt%
    \hbox{\usebox{\mcaption@ObjectBox}}%
    \vskip0pt%
  }%
}%
\sbox{\mcaption@CaptionBox}{%
  \if\mcaption@CaptionAlign t\vtop
  \else\vbox
  \fi
  {%
    \vskip0pt%
    \hbox{\usebox{\mcaption@CaptionBox}}%
    \vskip0pt%
  }%
}%
}%
\newcommand*{\mcaption@output@oddpage}{%
  \makebox[\linewidth][l]{%
    \usebox{\mcaption@ObjectBox}%
    \hspace*{\margincapsep}%
    \if\mcaption@CaptionWide t%
      \hspace*{-\margincapwidth}%
    \fi%
    \smash{\usebox{\mcaption@CaptionBox}}%
  }%
}%
\newcommand*{\mcaption@output@evenpage}{%
  \makebox[\linewidth][r]{%
    \smash{\usebox{\mcaption@CaptionBox}}%
    \if\mcaption@CaptionWide t%
      \hspace*{-\margincapwidth}%
    \fi%
    \hspace*{\margincapsep}%
    \usebox{\mcaption@ObjectBox}%
  }%
}%

\newcommand*{\mcaption@output@boxes}{%
  \if@twoside%
    \checkoddpage%
    \ifoddpage%
      \mcaption@output@oddpage%
    \else%
      \mcaption@output@evenpage%
    \fi%
  \else%
    \mcaption@output@oddpage%
  \fi%
}%
\endinput
%%
%% End of file `mycaption.sty'.

test.tex

\documentclass[a4paper,twoside]{book}

\usepackage{showframe}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{float}
\usepackage[demo]{graphicx}
\usepackage{etoolbox}
\usepackage{mycaption}
\usepackage{marginnote}
\usepackage{caption}
\usepackage{changepage}
\usepackage{calc}

\captionsetup{
  labelfont={bf},
  font=small}

\newlength{\normalwidth}
\setlength{\normalwidth}{\textwidth}

\newlength{\fullwidth}
\setlength{\fullwidth}{\textwidth}
\addtolength{\fullwidth}{\marginparsep}
\addtolength{\fullwidth}{\marginparwidth}

\newlength{\marginwidth}
\setlength{\marginwidth}{0pt}
\addtolength{\marginwidth}{-\marginparwidth}
\addtolength{\marginwidth}{-\marginparsep}

\newenvironment{widepage}{
	\begin{adjustwidth*}{}{\marginwidth}%
    \setlength{\textwidth}{\fullwidth}%
    \setlength{\linewidth}{\fullwidth}%
    \setlength{\columnwidth}{\fullwidth}%
}{\end{adjustwidth*}}

\renewenvironment{figure*}[1][htbp]{
	\begin{figure}[#1]%
		\begin{widepage}%
}{\end{widepage}\end{figure}}

\begin{document}

\lipsum[1]

\begin{figure*}[H]
	\begin{margincap}[wb]
		\includegraphics[width=\textwidth,height=200pt]{broken_loop}
		\caption{Wide bottom. Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure*}

\lipsum[2]

\begin{figure}[H]
	\begin{margincap}[b]
		\includegraphics[width=\textwidth]{broken_loop}
		\caption{Bottom. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure}

\newpage

\lipsum[3]

\begin{figure*}[H]
	\begin{margincap}[wt]
		\includegraphics[width=\textwidth]{broken_loop}
		\caption{Wide top. Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure*}

\lipsum[4]

\begin{figure}[H]
	\begin{margincap}[t]
		\includegraphics[width=\textwidth]{broken_loop}
		\caption{Top. Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure}

\newpage

\lipsum[5]

\begin{figure*}[H]
	\begin{margincap}[w]
		\includegraphics[width=\textwidth]{broken_loop}
		\caption{Wide (default align). Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure*}

\lipsum[6]

\begin{figure}[H]
	\begin{margincap}
		\includegraphics[width=\textwidth]{broken_loop}
		\caption{Default align. Some caption that is a bit longer than you might expect to see here, but that is OK. Some caption that is a bit longer than you might expect to see here, but that is OK.}
	\end{margincap}
\end{figure}

\lipsum[7]

\end{document}