If you must use book and cannot use titling, then you can redefine \maketitle.
I copied the definition from book.cls and added two lines:
\vskip 1.5em% Added this
{\Large \@subtitle \par}% and this
right below the title. I also added a wrapper command for adding the subtitle:
\def\subtitle#1{\gdef\@subtitle{#1}}
Without a MWE, this is the best one can do:

\documentclass{book}
\makeatletter
\def\subtitle#1{\gdef\@subtitle{#1}}
\def\maketitle{\begin{titlepage}%
\let\footnotesize\small
\let\footnoterule\relax
\let \footnote \thanks
\null\vfil
\vskip 60\p@
\begin{center}%
{\LARGE \@title \par}%
\vskip 1.5em% Added this
{\Large \@subtitle \par}% and this
\vskip 3em%
{\large
\lineskip .75em%
\begin{tabular}[t]{c}%
\@author
\end{tabular}\par}%
\vskip 1.5em%
{\large \@date \par}% % Set date in \large size.
\end{center}\par
\@thanks
\vfil\null
\end{titlepage}%
\setcounter{footnote}{0}%
\global\let\thanks\relax
\global\let\maketitle\relax
\global\let\@thanks\@empty
\global\let\@author\@empty
\global\let\@date\@empty
\global\let\@title\@empty
\global\let\title\relax
\global\let\author\relax
\global\let\date\relax
\global\let\and\relax
}
\makeatother
\begin{document}
\title{Main Title}
\subtitle{Subtitle}
\author{Me}
\date{\today}
\maketitle
\end{document}
Or, a less messy version with etoolbox's \patchcmd:
\documentclass{book}
\usepackage{etoolbox}
\makeatletter
\def\subtitle#1{\gdef\@subtitle{#1}}
\patchcmd\maketitle
{{\LARGE \@title \par}}
{{\LARGE \@title \par}%
\vskip 1.5em
{\Large \@subtitle \par}}
\makeatother
\begin{document}
\title{Main Title}
\subtitle{Subtitle}
\author{Me}
\date{\today}
\maketitle
\end{document}
scrbook(see this) or other class. Ifbookis required, then you can use thetitlingpackage (see this). Otherwise you could resort to redefining\maketitle. – Phelype Oleinik May 30 '18 at 23:25\maketitle? b) using t titling package? Otherwise, we can just guess because it's clearly a problem with your "specific book template", so probably yes, you'll have to change the.clsfile. – May 31 '18 at 00:04\renewcommand{\maketitle}{....}-- find the\maketitlecommand in your class, and change it (that's more or less what thetitlingpackage does). – May 31 '18 at 00:10