\documentclass{scrbook}
\usepackage{fontspec}
\setmainfont{OpenDyslexic}[
  Path = fonts/OpenDyslexic/,
  Extension = .otf,
  UprightFont = *-Regular,
  BoldFont = *-Bold,
  ItalicFont = *-Italic,
  BoldItalicFont = *-BoldItalic
]
%
\begin{document}
\input{./book/copertina.tex}
\input{./book/00_premessa.tex}
\end{document}