An update...I abandoned Facebook ads and Amazon campaigns, and tried Google AdWords Express. There are two options for google ads...the Express version and the full-featured version. I elected to use AdWords Express because it is user-friendly, and the full-featured version is more for marketing professionals, which I most certainly am not! I chose to try AdWords because I got a $100 credit by creating a commercial website through Squarespace. I had nothing to lose, so why not try it?
Well, I'm happy I did. I created a simple ad targeting specific phrases, like ebook, new book, etc. Google allows you to set a maximum average daily budget, and a total monthly maximum. It also tells you what people like you are spending on their campaigns, so you can gauge how you compare with the competition, so to speak. I started with $10 a day, and let it run. The results were nearly instantaneous...double the daily sales, and far more Kindle Unlimited page reads, at a cost of about 25-30 cents per click, less than Facebook and Amazon.
I'll be honest, I was taken aback by this. From what I had read, people were saying that Google's ads were too expensive compared to Facebook and Amazon. This certainly isn't my experience. I'm not sure that I'm brave enough to increase my marketing budget further...not yet, anyway...but I plan to continue to use Google AdWords Express.