Remove labels in printer friendly version of books
Here is how I approached the problem:
Copied

/modules/book/book-node-export-html.tpl.php
/modules/book/book-export-html.tpl.php


to my theme directory and modified the $contents variable via str_replace.

This works fine as a temporary fix, but I guess performance-wise this is not the best solution, would would be still nice to find a way to apply CSS.