Click on the 3 dots and click delete. It'll remove the folder from your list. If you have any books stored in the folder they'll go back into your all books collection.
You can also archive a book from your account by clicking on the 3 dots and clicking on "archive". The book will be removed from your account.
If you accidentally archive a book and need to get it back get in touch with us with the details and we can help restore the book to your account.