[HELP] Making Money With Free Book Downloads

I was wondering what would be the best method to earn income of a free ebook download?

I have few ebooks that I would like people to be able to download for free, but I would still like to make a profit from.
I was thinking an effective more would be to have them sign up for a newsletter/mailing list for them to get the free download, and then email them with promotions and affiliate links. Also, that I would send them a newsletter with content they would find of value, there for inceasing the likelyhood for them actually clicking the promotion/affiliate link.

Would this be a good method to make money from a free ebook download, or would there be a more effective way to profit from them?

Stuff the ebook with affiliate links/content lock for download/add ppi to download/ask donations/use "pay with a tweet" or "pay with a like" ..

While it is tempting to ask for email for ebook,if you give the book away as free download with no sign up and place an ad in the ebook that sends reader to an opt in page for a "better offer/method" you will get rid off alot of tirekickers and get better conversions when sending your emails/newsletter with affiliate links or better your own product links.

As an advise make sure you inquire about copyrights. That would be a shame you get into trouble mate

Thank you everyone I really appreciate the ideas! In regards to copyright, the books are either owned by myself or in the public domain.

Put some affiliate links in it and your website URL at the end. Then google "Pay with a tweet" and say that people have to tweet/share your website to be able to download the book.
If you use wordpress I recommend the "Social Bribe" plugin.