I was working on this site recently when I had a bit of a revelation: the people who read a book all of the way to the end are precisely the people most motivated to review, FB-post, Tweet, Pin, etc. about it, thus sharing their experience with their friends, a group of people who may never have heard of your book, but who will take seriously a mention by your reader.
Better than any advertising you could pay for!
And web sites have been taking advantage of this truth for years — which has led to the ubiquitous bars of buttons that make it as easy as possible to share a web page (or product or…) with your nearest and dearest.
My revelation was that, since an ebook is merely a highly specialized web page, I should include a set of just those sorts of links at the end of each book: links to review the book on the bookseller’s site, on Goodreads, or on my own site, or to post something about the book to Twitter, Facebook, or Pinterest.
Joel Friedlander posted my article on his wonderful resource for small and self-pubishers, The Book Designer, as Six Things You Should Be Including in Your Ebook (and Probably Aren’t)