I'm trying to add two menus to the Books tab.

Menus and items in menus are not the same thing. The whole bar is the menu. http://en.support.wordpress.com/menus/ The pages you add to the menu http://en.support.wordpress.com/menus/#adding-pages and sub-pages in drop-downs below them http://en.support.wordpress.com/menus/#changing-the-order-of-menu-items-and-creating-sub-menus are menu items.

Here’s a link to a custom menu walkthrough > http://onecoolsitebloggingtips.com/2011/08/11/wordpress-com-custom-menu-walk-through/

There are many common errors, misunderstanding and misconceptions when creating custom menus and you can read about them here > http://wpbtips.wordpress.com/2011/12/19/custom-menus/