\documentclass[12pt,letter]{book}      % Specifies the document class
\usepackage{geometry}
\begin{document}
\section*{IIII. If my complaints}
\label{sec:complaints}
\subsection*{Cantus}
\label{sec:complaintscantus}
\lilypondfile{complaints-1.ly}
\geometry{landscape}
\clearpage
\subsection*{Altus}
\label{sec:complaintsaltus}
\lilypondfile{complaints-2.ly}
\geometry{portrait}
\clearpage
\section*{V. Can she excuse my wrongs}
\label{sec:excuse}
\subsection*{Cantus}
\label{sec:excusecantus}
\lilypondfile{excuse-cantus.ly}

\end{document}