We can see that people love free books.
What I don't know is the correlation between downloading free books and sales. Are free books the BEST way to promo a new-to-the-public author? I don't know.
This 5 free days thing out of 90 is all that Amazon offers. I think it's pitiful. Amazon could have a page that changed every day where indie authors were featured, picked at random by computer. Or they could allow advertising. I'm sure they can figure out a lot of ways. They're just not doing it. That's fine. It is what it is. It's a venue we didn't have 4 years ago. Is it perfect? It's perfect for some. I'm trying to address the potential pitfalls.
If you have the freebie and the sales after, good for you. It's not worth it for me.
Here's this month's freebie. Before the freebie began a week ago, I had 1 sale. And you can see 1 return. There was 1 review from a verified purchaser.
Let me restate. There have been no sales since the free offering. 1 sale is the total. I made no money on this book. (The book that Amazon accused me of not writing and all the agitation that created.)
My best selling book is not on Kindle Select nor will it ever be. Does Kindle Select bump me into greater recognition and sales? No, it doesn't. There is no detectable increase in sales of anything on Kindle Select or that there's been a spill-over effect to books not in the program.
This is my experience of it. YMMV.