you will have to use a serverside language like php, asp or jsp.

You will want to make a member system where they log in. And check if they have bought anny e-books and have already downloaded them or not.
You might want to do this with a database like MySQL or MS ACCESS, oracle or an other type of database.

I don't have frontpage myself but since it's a wysiwyg editor I don't think that it is possible.

You will have to learn 1 of the serverside languages (or search for a script on google, hotscripts,...) and how they participate with a database.