Think of all the types of correspondence you might be typesetting within this novel... they may be very diverse. As such...
I've always been keen on avoiding the structured letter environment (and similarly, the default letter document class) for typesetting letters. The reason for this is because the setup within letter is usually pretty rigid: You need to define certain commands or else the compilation fails, and the spacing is fixed. If you're writing a single correspondence, then you can adjust these things to your needs, but when you're compiling a number of correspondences, you may want some flexibility that is readily available without fussing around. As a small example, it is more intuitive to write (say)
\begin{letter}
% OPENING
To whom it may concern: \par\medskip
% BODY
The body of the letter goes here.
% CLOSING
Sincerely, \par
Random Randofsky
\end{letter}
than to use the supplied macros
\begin{letter}{}
\opening{To whom it may concern:}
The body of the letter goes here.
\closing{Yours sincerely,}
\end{letter}
since you know exactly what's going on in the formatting and spacing (and designations can be supplied via comments, if needed). Moreover, did you know that the ordering of the commands \opening and \closing can be changed yet obtain the same output (due to the fact that the entire letter is actually set at the end of the letter environment).
So, I would suggest defining your own letter environment, and perhaps (depending on the scope of your letters) define some formatting commands for consistency. It allows you full control over setting of the letter contents. Even if it is as simple as
\newenvironment{letter}{\quote\itshape}{\endquote}