The most effective ads for selling on Amazon would almost certainly be Amazon ads ... but not if you only have a single novel published. For Amazon ads to be cost-effective, you need multiple books in a series before you start earning more in royalties than you're paying out for clicks. And note that popular search terms cost more per click. (You didn't mention how many books would be posted, so if you're releasing a bunch of books at once, definitely give Amazon ads a try. And Royal Road ads. You never know.)
On a funny note, when I went to your story page to check things out, my own story showed up on the "Others Have Also Liked (Beta)".