You are patching the wrong command. In the tufte-book
class, it is \@caption
and the relevant part is originally:
\@tufte@caption@font\@tufte@caption@justification%
\noindent\csname fnum@#1\endcsname: \ignorespaces#3\par%
see the file tufte-common.def
.

\documentclass{tufte-book}
\usepackage{wrapfig}
\usepackage[demo]{graphicx}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@caption}{\csname fnum@#1\endcsname: \ignorespaces#3}{\csname fnum@#1\endcsname}{}{}
\makeatother
\begin{document}
\begin{wrapfigure}{l}{4cm}
\begin{center}
\includegraphics[width=3cm]{graphic.pdf}
\caption[Caption for the list of figures]{}
\label{fig:figureX}
\end{center}
\end{wrapfigure}
\begin{wrapfigure}{l}{4cm}
\begin{center}
\includegraphics[width=3cm]{graphic.pdf}
\caption[Caption for the list of figures]{With caption}
\label{fig:figureX}
\end{center}
\end{wrapfigure}
\end{document}
This is easily adapted to test whether the caption is empty or not, and to center the caption:
\makeatletter
\patchcmd{\@caption}{\csname fnum@#1\endcsname:
\ignorespaces#3}{\Centering
\csname fnum@#1\endcsname\ifblank{#3}{}{: \ignorespaces#3}}{}{}
\makeatother

I hope you won't have to print a List of Figures in your document with such captions...
In such a case, remember to use the optional argument of \caption
:
\caption[short caption]{long caption}
Here are three alternatives.
First alternative:
\documentclass[oneside,DIV=17]{scrbook}
\usepackage[font={scriptsize,sf},labelfont={scriptsize,sf,bf}]{caption}
\usepackage[subrefformat=parens,labelfont={normalfont,scriptsize,sf},labelsep=space]{subcaption}
\setcapindent{0pt}
\begin{document}
\begin{figure}
\subcaptionbox{\label{fig:a}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:b}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:c}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:d}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:e}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:f}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:g}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:h}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:i}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:j}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:k}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\hspace{0.3\textwidth}
\caption{Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.
Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra metus rhoncus
sem. Nulla et lectus vestibulum urna fringilla ultrices.\\
\llap{\subref{fig:a}} Short text fit in one line\\
\llap{\subref{fig:b}} Short text fit in one line\\
\llap{\subref{fig:c}} Short text fit in one line\\
\llap{\subref{fig:d}} Short text fit in one line\\
\llap{\subref{fig:e}} Short text fit in one line\\
\llap{\subref{fig:f}} Short text fit in one line\\
\llap{\subref{fig:g}} Short text fit in one line\\
\llap{\subref{fig:h}} Short text fit in one line\\
\llap{\subref{fig:i}} Short text fit in one line\\
\llap{\subref{fig:j}} Short text fit in one line\\
\llap{\subref{fig:k}} Short text fit in one line
}
\end{figure}
\end{document}

Second alternative:
\documentclass[oneside,DIV=17]{scrbook}
\usepackage[font={scriptsize,sf},labelfont={scriptsize,sf,bf}]{caption}
\usepackage[subrefformat=parens,labelfont={normalfont,scriptsize,sf},labelsep=space]{subcaption}
\setcapindent{0pt}
\begin{document}
\begin{figure}
\subcaptionbox{\label{fig:a}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:b}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:c}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:d}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:e}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:f}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:g}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:h}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:i}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:j}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:k}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\hspace{0.3\textwidth}
\caption{Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.
Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra metus rhoncus
sem. Nulla et lectus vestibulum urna fringilla ultrices.\\
\protect\makebox[.5cm]{\subref{fig:a}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:b}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:c}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:d}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:e}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:f}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:g}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:h}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:i}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:j}} Short text fit in one line\\
\protect\makebox[.5cm]{\subref{fig:k}} Short text fit in one line
}
\end{figure}
\end{document}

Third alternative:
\documentclass[oneside,DIV=17]{scrbook}
\usepackage[font={scriptsize,sf},labelfont={scriptsize,sf,bf}]{caption}
\usepackage[subrefformat=parens,labelfont={normalfont,scriptsize,sf},labelsep=space]{subcaption}
\setcapindent{0pt}
\begin{document}
\begin{figure}
\subcaptionbox{\label{fig:a}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:b}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:c}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:d}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:e}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:f}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:g}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:h}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:i}}{\rule{0.3\textwidth}{0.25\textwidth}}\par
\subcaptionbox{\label{fig:j}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\subcaptionbox{\label{fig:k}}{\rule{0.3\textwidth}{0.25\textwidth}}\hfill
\hspace{0.3\textwidth}
\caption{Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.
Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra metus rhoncus
sem. Nulla et lectus vestibulum urna fringilla ultrices.\\
\protect\makebox[.5cm][l]{\subref{fig:a}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:b}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:c}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:d}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:e}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:f}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:g}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:h}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:i}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:j}} Short text fit in one line\\
\protect\makebox[.5cm][l]{\subref{fig:k}} Short text fit in one line
}
\end{figure}
\end{document}

Best Answer
Would this be what you seek? Use of
subfigure
environment fromsubcaption
packageUpdate: second attempt
Code
Code