When hooked to either the_content or the_excerpt filters, this should append the contents of the `author-box.php` file in your theme before the Jetpack ShareDaddy contents is added!

inc-author-box.php

<?php

/**
 * When hooked to either the_content or the_excerpt filters, 
 * this should append the contents of the `author-box.php` 
 * file in your theme before the Jetpack ShareDaddy contents 
 * is added, so long as the priority is 18 or less!
 * (as ShareDaddy's sharing options go in at priority 19)
 * 
 * @param string $content The post content as passed to the function
 * @returns string The content with the author box appended to it.
 */
function my_author_box( $content = '' ) {
	ob_start();
	get_template_part( 'author', 'box' );
	$author_box = ob_get_clean();
	return $content . $author_box;
}
add_filter( 'the_content', 'my_author_box', 18 );