With the XSLT parameter sty-file you can include an arbitrary LaTeX package instead of the default tbook-pl.sty. In this file you may change the default behaviour in a cleaner way than it is possible with a cfg file. But you have to pay attention to the following:
\ifgallery\else \RequirePackage[...]{geometry} \fi
See LaTeX cfg file, for a complete example.
You can give the name of the desired LaTeX package in the class attribute of your top-level element. So if you say
<book class="mystyle.sty"> ...
then mystyle.sty is included, and not the default tbook-pl.sty. The file extension .sty in the class attribute is mandatory, otherwise it's ignored in the LaTeX output. However, an explicit sty-file parameter on the command line has the highest priority.