Open the folder you want to delete, 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.

All Archived books will be saved in the "Archive" folder which is accessible at the bottom of your Books page:

From the Archive folder, click on the 3 dots below a book and you can restore it back to its original folder, or permanently delete the book. If you do delete a book, this cannot be undone.