Follow given steps to hide elements based in security groups:

1. Add a groups attribute on <header> to hide it from other users:

...
<header groups="my_library.group_library_user">
...

2. Add the groups attribute on the <menuitem> book category to display it only for librarian users:

    <menuitem name="Book Categories"
        id="library_book_category_menu"
        parent="library_base_menu"
        action="library_book_category_action"
        groups="my_library.group_library_librarian"/>

Restart the server and update the module to apply the changes.