There are programs that you add to your CGI folder that will allow your links to stay hidden, and PayPal will connect your purchaser to the link only after the payment has been accepted.
I'm presently using a service called Linklok from
Vibralogix. It allows for multiple books/links, you set prices (and can even set prices regionally), you set how long a link stays active/downloadable. It is a PayPal-authorized application, so you won't have trouble using it, and you pay for one app per site, but it'll be good for any number of digital products. The entire transaction is on automatic.
There are others available out there as well.