You have written your book and self published it well on KDP.
You have to choose between running Ads on Amazon or Facebook to maximize on sales.
My personal preference is running Amazon Ads.
Tell you what, people don’t move from one site to another easily..
You can’t share your blog post on Instagram and expect people to read it.
Why? They can’t start leaving Instagram to read a blog. Too much work.
Same applies to your book if you advertise it on Facebook and expect people to leave Facebook to check on a book on Amazon.
They won’t. You’ll have wasted your money.