I have written about a wonderful site for authors to promote their books before called ThatPartWhere. This site is a great advertising tool and relatively inexpensive. For ten dollars they will post a full page ad for one day on their site as well as on Facebook and it will be tweeted throughout the day. Also once the ad day has passed your ad will remain on their site indefinitely in their catalogue for anyone to peruse. You submit a scene or a chapter from your book and a description of the scene and they look after the rest!
If you click the above link, you will see what an ad can look like by having a look at my ad.