Right now, it's not possible to delete client folders.
However, a handy little tip is to rename folders you no longer need to something like "Archive" or "Inactive", and then actually use them to store your old books.
(Perhaps books you don't necessarily want to delete, but don't want clogging up your workspace).
To rename a folder, mouse over it and you'll see a small edit pencil icon. Click this and it'll take you to the edit folder options.