You’ll need to change this in, most likely, your header.php file in your themes root. I would suggest that you use something like YoastSEO Plugin which will give you granular manipulation of your sites <title> and <meta> tags. If this is not an option, you would have to dive into the header.php template and update it there.
For example:
<?php if( is_category( 'my-category-slug' ) ) : ?>
<title><?php _e( 'The Best Mystery Books' ) ; ?></title>
<?php else : ?>
<title><?php wp_title( '|', true, 'right' ); ?></title>
<?php endif; ?>
Make sure to check that the slug ('my-category-slug') matches your mystery book slug. You can see more about the arguments that can be passed to is_category() for more relevant matching.