Hi - I had a similar problem and just worked it out. You need to 'sell'your ebook so go to My Dashboard/My books, click on the book you want to share, scroll down and fill in the pricing information. Instead of clicking on FREE, I continued to fill in paypal details. Once this is complete, you have options to share it on facebook or a link appears down the bottom of the page so you can copy and email it.
Hope that helped.