main.tex

% We use part-chapter-section structure.
\documentclass{myclass}

% We load customized commands.
\usepackage{mypackage}

%==============================================================================
% Document.
%==============================================================================

\begin{document}

%------------------------------------------------------------------------------
% Front matters.
%------------------------------------------------------------------------------

\frontmatter

\maketitle

% Table of contents.
\tableofcontents

% Preface.
\subimport*{./}{preface.tex}
\addcontentsline{toc}{chapter}{Preface}

%------------------------------------------------------------------------------
% Main matters.
%------------------------------------------------------------------------------

\mainmatter

% All parts are in separated folders.
% We include them here.
\subimport{part1/}{main.tex}
\subimport{part2/}{main.tex}

%------------------------------------------------------------------------------
% Back matters.
%------------------------------------------------------------------------------

\backmatter

\printindex

\end{document}