Recommendation request for file delivery system

I am seeking recommendations for a WordPress plug-in or an online service that can help me automatically distribute computer files via e-mail.

I work for an NGO doing ‘literary campaigning’, and our site uses WordPress. Recently we have been creating e-Books and making them available as a free download.

Obviously we wanted to capture the e-mail addresses of everyone who downloaded it. I used a tweaked version of Contact Form 7 plug-in for WordPress to ‘reveal’ the download links after a person entered their e-mail address, but this was an inelegant method (see it in action here if you’re interested). If downloaders wanted to revist the page, they had to enter their address again. And there was no validation of e-mail addresses.

Are there any online tools that manage the distribution of documents or computer files? I need a simple system that will take a person’s e-mail address, log it for me. And then it either e-mails them a download link, or simply e-mail them the ePub file itself.

I have not been able to find a WordPress plug-in that does this, but I may have been using the wrong search parameters! I am not averse to using a third party service – it doesn’t need to be a WordPress plug-in. I imagine that the wider Open Source/shareware developer community must have some kind of solution in place for this task? Can anyone recommend such services? Obviously I have a preference for free services, as the projects I run are all not-for-profit!

Many thanks.