Adding the preface to contents

Add the line

\addcontentsline{toc}{chapter}{Preface}

Just after

\chapter*{Preface}

MWE

\documentclass{book}

\begin{document}
\tableofcontents
\chapter*{Preface}
\addcontentsline{toc}{chapter}{Preface}
\chapter{First}
\end{document} 

Output

If you want a different alignment like this one

use instead

\addcontentsline{toc}{chapter}{\protect\numberline{}Preface}

I'd modify \frontmatter and \mainmatter to do continuous (arabic) numbering and exploit the fact that chapters are not numbered in the \frontmatter.

\documentclass{book}

\makeatletter
\renewcommand{\frontmatter}{\cleardoublepage\@mainmatterfalse}
\renewcommand{\mainmatter}{\cleardoublepage\@mainmattertrue}
\makeatother

\begin{document}

\frontmatter
\tableofcontents

\chapter{Preface}

A preface

\chapter{Abstract}

An abstract.

\chapter{Introduction}

An introduction.

\mainmatter

\chapter{First}

\end{document}

It's much easier than remembering to add \addcontentsline.