This is a really brilliant blog from ex-marketing manager turned author 'Rose Red', about using social media to get publicity for your work. She really spells out the do's and don't's of the medium. Especially Twitter. She has excellent advice for both self-published and mainstream published writers. Apparently the knack is to be understated, funny, friendly and definitely not needy! Advertise indirectly and don't bombard people with requests to buy/read/look at your books. It definitely struck a chord with me - I've been put off several authors because they are tweeting every five minutes about their work. Do take a look at her blog, which is called Sprig Muslin and you can find it here:- http://sprigmuslin.blogspot.com/2012/01/its-all-about-socialising.html?spref=tw
Also some excellent advice on E-publishing and marketing at The Writers Guide to E-Publishing which you can find at http://thewritersguidetoepublishing.com/help-my-books-not-selling