Traditionally, to use Facebook as a book marketing tool you would make an author page, then set about getting as many likes as possible. In the long run, though, this plan is great for gathering your fans together in one place. Increased sales depend on finding new readers, so depending on a group of people who are already likely to buy your book doesn't do you any good. You've got to switch things up, and Facebook ads are the place to do it.
These are the targeted ads that show up on the right side of your Facebook feed page, the ones that magically match your interests, or at least the searches you've done online lately. Using these ads puts your books in front of thousands of fresh eyes, and all of them are already interested in your book's niche. They're predisposed to buy a book like yours.
Creating the Ad
These ads are simple -- one picture and a couple of short lines. Choose a beautiful picture that has something to do with your book topic. Show luscious fruit for a cookbook, a sexy man for a werewolf novel, a sunny beach for a summer read. Don't put in a picture of your book cover. You're selling curiosity here, not books. Reel them in with a personalized limited offer: FREE just for you today! or something similar. Finish them off with a tagline that makes them click. Want to meet a sexy wolf? or Do you have to sneak vegetables in your kids' meals?, etc. Something they can relate to. Don't be obvious in your selling. You just want them to be curious about the subject line, nothing else.
Change the sales URL to your website or your book sales page. Make sure they end up somewhere to buy your book as soon as they make that first curious click.
When you place the ad order, narrow down the interests of readers to drill down into your niche. You want it to be narrow enough so that only your potential fans will find it, but not so narrow that not enough people can click on the ad. A potential audience of around 300K is a good number. The number of days and amount you pay for the ad depends entirely on your budget. Keep in mind that Amazon, as well as other book sellers, rank your book according to sustained sales.
What that means is that you'll be ranked higher if you sell 20 books each day for 5 days than if you sell 100 books on one day before selling nothing else. Placing your ad for short periods of time on multiple days may be your best bet for this, and is a smart way to go.
Marketing a book is always a gamble, but this is one you can easily test. Create a small test ad and run it for a limited number of days -- one weekend, for example. Make sure you don't have any other promotions running, and that none of your books is newly published. Count the average sales pre- and post-sales, and compare the two to determine how well it works. Next weekend, try a new photo or tagline to see which one gets the best results.
Keep the Faith and may the Force be with You!