Password doesnt protect

[3books]

Hi, thanks to everyone that tried to help me last time I asked a question here.
I need some help urgently.
I have a website and I send people a link to download ebooks once they have paid.
I have protected the folder that holds the ebooks and when I send a link the recipient has to type in the username & password I supply.
I have noticed today that a lot more people are downloading the ebooks than have paid. After several hours of head scratching I have discovered that although by following a link to the actual item for download results in a request for password etc.
It is possible to type www.DOMAIN/FOLDER/ and you will go to the index of that folder then by clicking on the title of any ebook it will download.
I have had a couple of hundred ebooks too many downloaded just this month.
I need to protect the folder in question so that no one but me can view the contents or download anything without a username & password.
Any help would be appreciated

[spufi]

Is there not an index page for that folder? If not, put one there which should elimate a lot of the downloading. Beyond that, have the index page ask for the password, and username and then go from there.