I think some technique of backgroud passing of a password(hidden from the purchaser) can be used! In which, the password passes only if the user is on a page that he sees after buying the product. And even this technique is hidden i.e. the purchaser will hav no clue that a password was passed in the background. In this way, the book atleat cant be donwloaded from the website in an illegal manner!!
Thats what called "Maintaining a State" and thats where "Session Variables and Cookies" come in.

With PHP, you do so by :
PHP Code:
$_Session["loggedIn"] = true/false
setcookie("user")("loggedIn") = true/false 
With ASP :
Code:
Session("loggedIn") = true/false

Response.Cookies("user")("loggedIn") = true/false
I hope this helps.


- :S: