WordPress: Adding a Taxonomy Filter to Admin List for a CPT

// Adding a Taxonomy Filter to Admin List for a Custom Post Type
add_action( 'restrict_manage_posts', 'my_restrict_manage_posts' );
function my_restrict_manage_posts() {

    // only display these taxonomy filters on desired custom post_type listings
    global $typenow;
    if ($typenow == 'books_ibm') {

        // create an array of taxonomy slugs you want to filter by - if you want to retrieve all taxonomies, could use get_taxonomies() to build the list
        $filters = array('genre', 'writer');

        foreach ($filters as $tax_slug) {
            // retrieve the taxonomy object
            $tax_obj = get_taxonomy($tax_slug);
            $tax_name = $tax_obj->labels->name;

            // output html for taxonomy dropdown filter
            echo "<select name='$tax_slug' id='$tax_slug' class='postform'>";
            echo "<option value=''>Show All $tax_name</option>";
            generate_taxonomy_options($tax_slug,0,0);
            echo "</select>";
        }
    }
}

function generate_taxonomy_options($tax_slug, $parent = '', $level = 0) {
    $args = array('show_empty' => 1);
    if(!is_null($parent)) {
        $args = array('parent' => $parent);
    }
    $terms = get_terms($tax_slug,$args);
    $tab='';
    for($i=0;$i<$level;$i++){
        $tab.='--';
    }
    foreach ($terms as $term) {
        // output each select option line, check against the last $_GET to show the current option selected
        echo '<option value='. $term->slug, $_GET[$tax_slug] == $term->slug ? ' selected="selected"' : '','>' .$tab. $term->name .' (' . $term->count .')</option>';
        generate_taxonomy_options($tax_slug, $term->term_id, $level+1);
    }

}