\documentclass[11pt,twoside]{book}\makeatletter

\usepackage{fontspec}
\usepackage{xunicode}
\setmonofont{DejaVu Sans Mono}
\setromanfont{Minion Pro}
\setsansfont{Myriad Pro}
\begin{document}
x\textsf{b}\texttt{y}
\end{document}