library/library_app/views/library_menu.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!-- This Menu Item will appear in the Upper bar, That's why It needs NO parent or action -->
<odoo>
    <menuitem id="menu_library" name="Library"/>

    <act_window id="action_library_book"
        name="Library Books"
        res_model="library.book"
        view_mode="tree,form"/>

    <menuitem id="menu_library_book"
        name="Books"
        parent="menu_library"
        action="action_library_book"/>
</odoo>