How to set not italic or not bold?

You can use \normalfont

\documentclass{book}
\begin{document}
 \textit{Some italic and {\normalfont non italic} text}

 \textbf{Some bold and {\normalfont non bold} text}

 \emph{Some italic and {\normalfont non italic} text}
\end{document}

For the purpose of this discussion, I will assume that you use either \textbf{...} or {\bfseries ...} to generate bold text, and either \textit{...}, \emph{...}, or {\itshape ...} to generate italic text. These methods can be combined to get bold italic text, viz., \textbf{\textit{...}} or {\bfseries\itshape ...}.

• To cancel/override just the italic font shape, while leaving the (bold or non-bold) font weight unchanged, use either \textup{...} or {\upshape ...}:

    \textit{Some italic and some \textup{non italic} text}
  
    {\itshape Some italic and some {\upshape non italic} text}
  

  The \emph{...} method can be nested:

    \emph{Some italic and some \emph{non italic} text}
  

  Note that whereas the \textit{...} and \emph{...} methods take care to insert a so-called italic correction in the transitions from italic to non-italic text, the {\upshape ...} method does not.

• To override just the bold font weight, while leaving the (italic or upright) font shape unchanged, use either \textmd{...} or {\mdseries ...}:

    \textbf{Some bold and some \textmd{non bold} text}
  
    {\bfseries Some bold and some {\mdseries non bold} text}
  

• To force both normal font weight and normal font shape simultaneously, the preceding methods can be combined. Unless the font face default has been reset earlier (to \ttfamily or \sffamily, say; see the comment by @JonathanLandrum below), one can also use either \textnormal{...} or {\normalfont ...} to force both normal font weight and shape:

    \textbf{\textit{Some bold-italic, \textmd{non-bold italic}, \textup{non-italic bold}, and \textmd{\textup{{upright\slash non-bold} text}}.}
  
    {\itshape\bfseries Some bold-italic, {\mdseries non-bold italic}, {\upshape non-italic bold}, and {\mdseries\upshape upright\slash non-bold} text.}
  

---

The results generated by these methods are illustrated in the following MWE:

\documentclass{article}
\usepackage{geometry}
\begin{document}

Some normal, i.e., upright and non-bold, text.
 
\medskip
\textit{Some italic and some \textup{upright} text.}

\emph{Some italic and some \emph{upright} text.}

{\itshape Some italic and some {\upshape upright} text.}


\medskip
\textbf{Some bold and some \textmd{non-bold} text.}

{\bfseries Some bold and some {\mdseries non-bold} text.}

\medskip
\textbf{\textit{Some bold-italic, \textmd{non-bold italic}, \textup{non-italic bold}, and \textmd{\textup{upright\slash non-bold}} text}.}

{\itshape\bfseries Some bold-italic, {\mdseries non-bold italic}, {\upshape non-italic bold}, and {\mdseries\upshape upright\slash non-bold} text.}

\end{document}