how to improve titlepage. Lines too close to each others

Question

I find the lines are too close to each others in the titlepage I am making.

I am not sure the correct way to fix this. Here is MWE

\documentclass[12pt,oneside]{book}
\begin{document} 

\begin{titlepage}
\begin{center}
\vspace*{0.1in}
\begingroup
\fontsize{16pt}{12pt}\selectfont
\textbf{A Solution Manual For}\\
\vspace*{0.5in}
\fontsize{28pt}{22pt}\selectfont
\textbf{Differential equations and the calculus of variations by L. ElSGOLTS. MIR PUBLISHERS, MOSCOW, Third printing 1977.}
\endgroup
\end{center}
\end{titlepage}
\newpage\null\thispagestyle{empty}\newpage

This is the text

\end{document}

Compile with lualatex gives

I think the problem is caused by increasing the font size for the book name but the default line spacing remains the same as before. May be there is a way to automatically adjust line spacing just for the title corresponding to the new font size?

What is the correct way to change font size and corresponding line spacing at same time?

I'd like a solution that keeps using titlepage as above and not other package or the normal maketitle command for other reasons.

TL 2023

Answer 1

The second argument is the distance between lines, so it should be larger. We also use \par-like command called \endgraf. For example:

\documentclass[12pt,oneside]{book}
\begin{document} 

\begin{titlepage}
\begin{center}
\vspace*{0.1in}
\begingroup
%\fontsize{16pt}{12pt}\selectfont % PS
\fontsize{16pt}{20pt}\selectfont % PS
\textbf{A Solution Manual For}\\
\vspace*{0.5in}
%\fontsize{28pt}{22pt}\selectfont % PS
\fontsize{28pt}{34pt}\selectfont % PS
\textbf{Differential equations and the calculus of variations by L. ElSGOLTS. MIR PUBLISHERS, MOSCOW, Third printing 1977.}
\endgraf
\endgroup
\end{center}
\end{titlepage}
\newpage\null\thispagestyle{empty}\newpage

This is the text

\end{document}