Data won't save on a straightforward metabox, what am I missing?

Question

I need a second set of eyes, because its late and I can't see this anymore.

Why won't this save? What am I missing?

add_action("admin_menu", "tf_book_stuff_create");

function tf_book_stuff_create(){
    add_meta_box('tf_book_stuff', 'Book Stuff', 'tf_book_stuff', 'books');
}

function tf_book_stuff() {
    global $post;
    $custom = get_post_custom($post->ID);
    $tf_book_number = $custom["tf_book_number"][0];
    $tf_book_reader = $custom["tf_book_reader"][0];

    echo $tf_book_isbn;
    echo $tf_book_level;

?>  

    <div class="admin_meta"> 
    <ul>
        <li><label>Book ISBN:</label><input name="tf_book_number" value="<?php echo $tf_book_number; ?>" /></li>
        <li><label>Reading Level:</label><input name="tf_book_reader" value="<?php echo $tf_book_reader; ?>" /></li>
    </ul>
    </div>
<?php }



add_action("admin_menu", "save_tf_book_stuff");
function save_tf_book_stuff($post_id) {
    global $post;

    update_post_meta($post->ID, "tf_book_number", $_POST["tf_book_number"]);
    update_post_meta($post->ID, "tf_book_reader", $_POST["tf_book_reader"]);
}

Thanks

Answer 1

Instead of using the global $post variable, try just using the $post_id that was passed to the function in the first place.

update_post_meta($post_id "tf_book_number", $_POST["tf_book_number"]);
update_post_meta($post_id, "tf_book_reader", $_POST["tf_book_reader"]);

Answer 2

Oh I found it. Its probably a little backwards (ie I'm sure there are shorter nicer ways to do it), but it works.

function save_tf_book_stuff() {
global $post;
$custom = get_post_custom($post->ID);
update_post_meta($post->ID, "tf_book_number", $_POST["tf_book_number"]);
update_post_meta($post->ID, "tf_book_reader", $_POST["tf_book_reader"]);

}