I'm not sure if this is just my installation or not but when I set a user to the Author that user can add a new series through a bug.
If the author goes to the create a new post page in the admin panel, the sidebar widget still displays the textbox and "add" button for a new series. When this author clicks "add" there is an error message saying "Sorry but you don't have permission to add series." Unfortunately, if you refresh the page, tada the new series has been added.
Thanks a lot for the great plugin, I'm really enjoying using it.