The book's name at the top of the page is not filtered, even with a filter set to apply to content and headings. This is not consistent given the filtering is performed when the book's name is included in the page's title, eg:

$PAGE->set_title($pagetitle);

My proposed change, line 209 of mod/book/view.php

echo $OUTPUT->heading($book->name);

echo $OUTPUT->heading(format_string($book->name));

With this modification, book names using the multilingual filter (filter_multilangsecond) markup is displayed properly.