There is no Delete menu item in the notebook menus in order to avoid accidental notebook deletion if you confuse it with a note.
You can delete notebook from its Properties page. First you need to click on "Properties" menu item in the notebook menu:
Then you need to click "Delete" button at the bottom of the notebook properties page:
Please note that a backup will be created automatically at the moment you delete your notebook, therefore you can easily restore it in the future if you need.