\documentclass{book}
\usepackage[layout=letterpaper,margin=1in]{geometry}
%\usepackage[linktoc=all]{hyperref}

\usepackage{fancyhdr}

\usepackage{etoc}

\newcommand{\etocsettocmargins}[3][0pt]{%
\renewcommand{\etocaftertitlehook}
  {\let\oldpar\par\def\par{%
   \advance\leftskip by -#1\relax
   \advance\leftskip by #2\relax
   \advance\rightskip by #3\relax\oldpar}}}

\newcommand{\etocresettocmargins}{%
        \renewcommand{\etocaftertitlehook}{\relax}}

\begin{document}

\fancyfoot[L]{Revision 0.2.3 -- Dec 18, 2012}
\pagestyle{fancyplain}

\etocsettocmargins{.1\linewidth}{.1\linewidth}
\etocsettocstyle
    {\section*{\hskip.1\linewidth\contentsname}}
    {}

\tableofcontents

\part{FIRST PART}
\section{Small Section but which has a long name to see if it works, actually it
has a very very long name}

\part{SECOND PART}

\chapter{Chapitre 1}

\section{Huge Section}

Here is the TOC for this section:

\etocsettocstyle{\centering\subsection*{Contents of this section}}{}
\etocsettocmargins[1.5em]{.275\linewidth}{.275\linewidth}

\localtableofcontents

\subsection{Large Subsection 1}

This should be the TOC for just this {\bf subsection}. Its entries are centered
with the left and right margins and equal to 1/20th of the line width.

\etocsettocmargins[3.8em]{.05\linewidth}{.05\linewidth}
\etocsettocstyle
    {\subsubsection*{Contents of this subsection}\hrule\smallskip}
    {\nobreak\medskip\hrule\bigskip}

\localtableofcontents

\subsubsection{Thing 1}
1
\subsubsection{Thing 2}
2
\subsubsection{Thing 3 also has a very very very long name even longer than
  would be reasonable let's make it even longer}
3

\subsection{Large Subsection 2}

This should be the TOC for just this {\bf subsection}. The technique of the
minipage is the simplest:

\begin{center}
\begin{minipage}[c]{0.6\linewidth}
 \etocsettocmargins[3.8em]{0pt}{0pt}
 \localtableofcontents
\end{minipage}
\end{center}

\subsubsection{Thing A also has a very very very long name even longer than
  would be reasonable and truly it is a very very long name}
A
\subsubsection{Thing B}
B
\subsubsection{Thing C}
C

\end{document}