Open the folder, then click on the 3 dots in the top right and then 'Delete folder'. It'll remove the folder from your list. If you have any books stored in the folder then they'll still be accessible from 'All books'.

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 then head to the 'Archive' folder at the bottom of your folders list. From there you can restore the book.