\documentclass[a4paper,11pt,fleqn]{book}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{newtx}
%...

\title{Wprowadzenie do \LaTeX a}
\author{Jan Techowski}
\date{2025}

\begin{document}
\maketitle
\frontmatter
\setcounter{page}{5}
\tableofcontents
\cleardoublepage

\mainmatter
\include{wprowadzenie} 
\include{tekst} 
\include{tabele} 
\include{matematyka}
\include{podsumowanie}
\appendix
\include{overleaf}
\include{texstudio}

\backmatter
\include{polecenia}
\bibliographystyle{plain}
\bibliography{latex}
\end{document}