Amazon Kindle has their own built-in ad platform,and it’s super effective but since they pay authors 2 months later for sales and they require a $100 commitment,you need to either set very short ad run times or risk that $100 that you might not see back for a while. On the brighter side,you can bid as low as 15 cents per click and I’ve never had a book not sell enough copies to make this effective. So it’s a good thing. Just don’t spend money you don’t have yet since you won’t see those royalties for 2 months or more.
Kindle ROI –this one is a Chrome browser plugin that sits to the left of my address bar. I use it in conjunction with the KDSpy tool (from the same creators) that shows me how profitable other authors are. I’ve actually used the KDSpy tool to create a spreadsheet I can use to study the success of authors I find making $10k or more per month. Watching their marketing habits is one of the reasons I like the Kindle ROI tool because they all seemed to be using the newsletter submission tools while they built their own email lists. KDROI has 31 email newsletters that you can submit to with just one form. One thing I wasn’t prepared for when I first started using this was that even though it takes only 15 seconds to fill out the form,you have to go straight away back to your email account to confirm many of the listings. It’s still easier than hunting down the newsletters and using their submit form and figuring it out alone. I have since subscribed to the relevant email newsletters and I’m using them to find books and authors whose success I’d like to emulate.
Facebook ads are also effective. Choose your audience wisely,and very narrowly. There’s no limit to the number of ads you can run at once and you can really learn a lot about your readers by making audiences very specific. For example,if you’re writing horror you can chose to advertise only to members who like Steven King or something. You can also choose an audience based on age,income,location,favorite TV show…
And finally,as always,your OWN Facebook author page should not be overlooked,or your twitter account. Some authors seem to enjoy joining Facebook promotion groups (or maybe those are just fiverr people getting paid to post your link on a bunch of facebook groups) either way,I find these ineffective in the least,annoying as hell at the worst. Unless you’re writing about writing,the audience for your book is probably NOT a bunch of other writers who are also trying to sell their books,so groups whose sole purpose is self-promotion are really stupid IMHO.