How do I set the Verbatim font (or the mono font) to bold by default?

Just load the Courier New Bold font as the mono font. I've removed your Ligatures and Numbers options since these aren't available in this font (they produce warnings.)

\documentclass[11pt,oneside]{book}
% preamble
\usepackage{fontspec}
\setmonofont[Scale=0.8]{Courier New Bold}
% more preamble
\usepackage{verbatim}
\usepackage{fancyvrb}
% final part of eh preamble
\begin{document}
\begin{Verbatim}[commandchars=\\\{\}]
this text is in a monospace font
\end{Verbatim}
\end{document}

An alternative version that loads the Black (or ExtraBold, etc.) weight as “bold”, and also sets up italics:

\setmonofont{Some Font}[
  Scale = MatchLowercase ,
  UprightFont = * Bold ,
  ItalicFont = * Bold Italic ,
  BoldFont = * Black ,
  BoldItalicFont = * Black Italic ]

Change this to match the names of your font files. For example, if you have Some Font-Bold.otf and Some Font-ExtraBold.otf on your system, you would instead load UprightFont = *-Bold, BoldFont = *-ExtraBold, and Extension = .otf.