I have been promoting my book with #AMS ads for the past 4 years.

They have worked, but they haven't been cheap. I went way over budget, but I was determined to get my book in front of as many readers as possible. I learned a lot of things the hard way, and I made many costly errors along the way. After a couple of years of trying to figure them out, I finally found a list of key words that were profitable. Here is how I did it:
1. Create two AMS ads
2. Add 1,000 key words each (create your own and don't use suggested key
words unless you feel confident, they will drive readers to your book)
3. Create a budget for each ad.
4. Run both for six weeks.
5. See which words work, and delete the ones that are too expensive.
6. Then create a new ad using the key words that were profitable. When you finally collect a couple of hundred key words that worked, I shut the other ads down.
7. Get rid of everything else.
8. Delete key words costing too much.
9. Delete key words that aren't profitable.
10. I only allowed KDP to adjust my bid downward. I spent too much money allowing them to go up or down. Be careful if you try the up and down. I spent a ridiculous amount of money on ads with no return on my investment. Some day when you become more skilled (if that's possible), with the KDP ads, you can give it a try, but check your ads EVERY DAY.
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SUMMARY: KDP Ad Campaigns can cost a lot of money if you don't know what you are doing. They charge your credit card and keep it on file for all future ads. I think the costs have increased significantly over the past few years.
My advice: Tread lightly, unless you have help and guidance from a LIVE person (not a video), that guarantees that they can make you money by using this method of advertising. You should monitor your ad daily, to make sure you don't end up broke or in debt.
------------------------------------------------------------------------------------------------ UPDATE: July 11, 2021
I have used software programs, watched YouTube videos, read courses on Amazon Advertising and reviewed sites that focus on the subject. I spent nearly four years analyzing every Keyword that has ever generated a single sale. Based on my experience, I have listed below some of the things I have done to help make my ads more profitable:
I no longer use the Suggested Bid. I use the CPC and increase it by $.05. So, if the CPC is $.50, my bid is $.55.
If it is a new Keyword that does not yet have a CPC, I use $.60. Once it has a CPC, I immediately change it to $.05 above the CPC as indicated in #1. NOTE: I selected $.60 because it was an average number based on my sales divided by my sold units. I added the $.05 and therefore $.60 became my best guestimate. You can choose whatever amount you think would work for you. $.60 may seem high to some authors.
If I have spent $10.00 on any Keyword, and it has not generated a single sale, I delete the Keyword.
If any Keyword has more than 10,000 impressions and no sales, I delete the Keyword.
If any keyword has an ACOS of 100 or higher, I delete the Keyword. I have kept the Keywords with an ACOS of 100 or lower, because I generate many sales with the ones between 80 and 100 ACOS. They are more expensive, but the Keywords that are much less expensive have lowered my average ACOS.
I no longer pay attention to the number of Clicks. I have many Keywords that have had one click and one sale or two clicks and two sales. If I were to delete the Keywords because it only had one Click, I would not have generated those sales. I no longer focus on the Clicks column.
I no longer pay attention to the CTR. Just like the Clicks column, I have some Keywords that have a CTR of .005 and it generates sales routinely. I have some with very high CTR and they do as well. However, many do not. So, I no longer focus on the CTR column.
I use the BROAD Match Type for new Keywords. Once a Keyword is generating sales, I add the PHRASE Match Type. I have not had as much success with the EXACT Match Type. PHRASE Keywords are often less expensive than the BROAD Match Type.
When working on my Ad bids, I double click the COLUMNS button. I uncheck everything except the CPC. Click the CPC heading and use the high to low CPC. It makes it easier for me to see which one of my bids needs to be adjusted upward or downward.
When working on my Ad Impressions & Spend, I double click the COLUMNS button. I uncheck everything except the Impressions & Spend columns. Click each heading and use the high to low versions. It makes it easier for me to see which one of my Keywords needs to be deleted because I am spending more than $10.00 or it has more than 10,000 Impressions with no sales.
Finding Keywords:
The Keyword Multiplier Tool has been helpful in finding variations of Keywords that have been generating sales for me: https://cleverclicksdigital.com/au/keyword-multiplier-tool-for-seo-ppc/
Variations of my genre Keywords,
Frequently bought together,
Customers who read this book also read,
Category Keywords,
Top 100 Books in my genre,
Any book that lists your book in the "Customers who bought this item also bought",
Add the PHRASE version of your Keyword once you make sales in the BROAD version,
Use Keywords that have generated sales listed in "Search Terms",
Keep a close watch on your "Search Terms". If readers are searching for things totally unrelated to your book, that is costing you money every time they click on them, add those Keywords to your Negative Targeting Keywords. Be careful not to add important Keywords that would be ones that would generate sales for your book.
I added words to my Negative Targeting list such as: sexual abuse, alcoholism, drug trafficking, anorexia, art, artists, address books, anxiety, celebrity, etc. I don't want to pay for clicks for words that have absolutely nothing to do with my book and that are costing me money. I update this list monthly.
I can't guarantee that the above tips will work for you. However, I do hope that they help!