Example migrated from Codex:

Setting Existing Values

Populating inputs with existing values takes a bit of trickery. One can’t simply access the $post global because when this action is run, $post refers only to the last post (the quick edit inputs are created only once, and cloned as needed when quick editing a column). There are two parts: storing the data on the page and hooking inlineEditPost.edit to set the input values. If the quick-edit columns are also displayed as custom columns, the data is already in the table. Otherwise, it can be added as hidden elements to an existing custom column.

Hooking inlineEditPost.edit must be done in a JS script, loaded after js/_enqueues/admin/inline-edit-post.js. The original method is saved and replaced with a new one which calls the original. This particular technique is a simple example of aspect-oriented programming.

PHP:

/* load script in the footer */
if ( ! function_exists('wp_my_admin_enqueue_scripts') ):
function wp_my_admin_enqueue_scripts( $hook ) {

	if ( 'edit.php' === $hook &&
		isset( $_GET['post_type'] ) &&
		'book' === $_GET['post_type'] ) {

		wp_enqueue_script( 'my_custom_script', plugins_url('scripts/admin_edit.js', __FILE__),
			false, null, true );

	}

}
endif;
add_action( 'admin_enqueue_scripts', 'wp_my_admin_enqueue_scripts' );


/* example of how an existing value can be stored in the table */
add_action( 'manage_book_posts_custom_column' , 'custom_book_column', 10, 2 );
function custom_book_column( $column, $post_id ) {
    switch ( $column ) {
      case 'inprint':
        // the !! means translate the following item to a boolean value
        if ( !!get_post_meta( $post_id , 'inprint' , true ) ) {
            $checked = 'checked';
        } else {
            $checked = '';
        }
        echo "<input type='checkbox' readonly $checked/>";
        break;

      case 'book_author':
        # ...
    }
}

scripts/admin_edit.js:

(function($) {

	// we create a copy of the WP inline edit post function
	var $wp_inline_edit = inlineEditPost.edit;

	// and then we overwrite the function with our own code
	inlineEditPost.edit = function( id ) {

		// "call" the original WP edit function
		// we don't want to leave WordPress hanging
		$wp_inline_edit.apply( this, arguments );

		// now we take care of our business

		// get the post ID
		var $post_id = 0;
		if ( typeof( id ) == 'object' ) {
			$post_id = parseInt( this.getId( id ) );
		}

		if ( $post_id > 0 ) {
			// define the edit row
			var $edit_row = $( '#edit-' + $post_id );
			var $post_row = $( '#post-' + $post_id );

			// get the data
			var $book_author = $( '.column-book_author', $post_row ).text();
			var $inprint = !! $('.column-inprint>*', $post_row ).prop('checked');

			// populate the data
			$( ':input[name="book_author"]', $edit_row ).val( $book_author );
			$( ':input[name="inprint"]', $edit_row ).prop('checked', $inprint );
		}
	};

})(jQuery);

Example migrated from Codex:

Saving Data

Data entered in custom inputs can be saved by hooking the save_post action.

add_action( 'save_post', 'save_book_meta' );

function save_book_meta( $post_id ) {
    /* in production code, $slug should be set only once in the plugin,
       preferably as a class property, rather than in each function that needs it.
     */
    $slug = 'book';
    if ( $slug !== $_POST['post_type'] ) {
        return;
    }
    if ( !current_user_can( 'edit_post', $post_id ) ) {
        return;
    }
    $_POST += array("{$slug}_edit_nonce" => '');
    if ( !wp_verify_nonce( $_POST["{$slug}_edit_nonce"],
                           plugin_basename( __FILE__ ) ) )
    {
        return;
    }

    if ( isset( $_REQUEST['book_author'] ) ) {
        update_post_meta( $post_id, 'author', $_REQUEST['book_author'] );
    }
    # checkboxes are submitted if checked, absent if not
    if ( isset( $_REQUEST['inprint'] ) ) {
        update_post_meta($post_id, 'inprint', TRUE);
    } else {
        update_post_meta($post_id, 'inprint', FALSE);
    }
}