\documentclass{article}

\usepackage{mfirstuc}

\begin{document}

\makefirstuc{abc}.

\makefirstuc{{\em abc}}.

\makefirstuc{\emph{abc}}.

\makefirstuc{\ae bc}.

\makefirstuc{{\ae}bc}.

\newcommand{\abc}{abc}%
\xmakefirstuc{\abc}.

\capitalisewords{a book of rhyme.}

\capitalisewords{a book\space of rhyme.}

\newcommand{\mytitle}{a book\space of rhyme.}
\capitalisewords{\mytitle}

\xcapitalisewords{\mytitle}

\end{document}
