Hello, Chris.
Using \\ to insert line breaks in the title should work okay. (If it
doesn't, let me know.)
When you write \title{My long\\book title}, the following happens:
a. The \thanklesstitle macro is set to the same contents as what you
put in \title except any \thanks, \sidenote, and \footnote macros have
been stripped away.
b. The \plaintitle macro is set to the same contents as \thankless
title if you don't provide an optional argument to title. If you do
provide an optional argument (e.g., \title[My shorter title]{My longer
title}), then it will be set to optional argument.
Finally, when we create the running heads, we replace \\ and \newline
with \space.
As for problems with Pandoc parsing titles with \\ or optional
arguments, I could see where that may be troublesome. Unfortunately, I
don't know how to get around that. (Such is the difficulty with
parsing LaTeX code – you really do need to parse *everything* to know
what's going on.)
If you encounter any trouble in the future or if you have any further
questions, please feel free to contact us.
—Kevin Godby