Facebook offers a great number of groups to promote your books.  If you have not already done so, you need to try this promo method.  Here is the basic formula.
Go to a promo book page. Post to the group a short book blurb and link to your book. I always use the Amazon link as it produces a nice pic of your book cover. Also, if the promo group offers sales/discussion posts, always pick discussion.Post no more than one book a day to a group. Don’t overdo your welcome.Pay attention to the group’s rules. Some are wide open but others have requirements. For example, “Kindle Unlimited” must have #KU, Kindle Unlimited, or something like that in the post, where it can be easily seen.Rotate through the groups. Using the list below, you should be able to do a month of promos for a book before any repeats.I track everything on a spreadsheet.  One sheet per book.  I note the post date for each promo group. I also keep my book blurb on the same sheet for that book.~~~~~~
Here is the blurb for Writing the Killer Mystery: Places, Clues and Guilt, Volume Four.
★★ Read for free on Kindle Unlimited ★★ ✔ 99 Cents with Kindle Matchbook☺ Now an Amazon Bestseller➭ Create dynamic settings➭ Invent great clues and evidence➭ Find means and motive for the killer✔ $2.99 to Buyhttps://www.amazon.com/dp/B07HM67D1Q/

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The post looks like this on FB.

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My list of promo groups on Facebook follows.   Remove any that don’t work for you. As you go on you can add to the list. I suggest sticking to groups with a few thousand follower or more.  Note some very similar names so use them as copied here. You can find these groups by doing a FB search on the name.
<Kindle Book Promotion> Authors, Writers & Bloggers-->>Promote Kindle Book ++All Things BooksAuthor/Publisher/Editor/Book ReadersAuthors Ebook promotion & Reviews , Amazon kindle self publishingBook Junkie PromotionsBOOK REVIEW & PROMOTIONCelebrating AuthorsEverything and Anything to do with BooksFree Author Book PromotionIndie Author Book Promotion PageKindle Amazon Book Club - Authors Promote Here!Kindle UnlimitedKindle Unlimited BookwormsKindle, Kindle, and KindlePassion for BooksPromote Your Book HerePromote Your Book Here!Promote Your Books HereThe Kindle Book LibraryUnlimited Kindle ReadsEBook PromotionsAuthors promote your books.Unlimited Book PromosKindle Amazon Book Club - Authors Promote Here!Book PromotionKindle/Book ClubGood Kindle Unlimited ReadsKindle Book Library-->> PROMOTE Kdbook! FREE!eBook & Book Ads and Promotions

Happy promoting!!!