My experience with free days is, that you are best using free days to drive sales to your OTHER books.
People who download your free book will come back to buy your OTHER books.
A lot of authors send me questions asking how I get my sales. But my key to success, is the fact that I have 170+ titles and write sets or series or serials. And not just fiction. Even my non-fiction is that way too.
For fiction I write as though I was writing a tv show. So that each story stands on it's own, but each story tells another segment of the character's lives. Like the Big Bang theory or The Simpsons. You can watch any episode and get a complete story without ever watching more. I do my fiction like that. That way I can write lots of stories about the same set of characters. Think of it like the Nancy Drew books, where each volume is about Nancy and her adventures, but each volume is a full complete story. (NOTE: DO NOT write a novel and break it up selling it one chapter at a time - that is against Amazon's ToS, and if they catch you doing it they will not only ban your books, they may also delete your Amazon account and block your isp.)
For fiction, I strive for stories of AT LEAST 50 pages (15,000 words), and aim at at least 75 to 90 pages, and I will point out that my best sellers are always the ones with 150+ pages (50,000+ words). My top selling fiction, is a series of novels, each volume being about 190/200 pages long (60,000 to 75,000 words). The shorter a story is, the more difficult I find it is to sell it.
In non-fiction I strive to focus on a narrow topic, within a broader topic. I'll write one big book (400 pages - 300,000 words or so) and then I'll divide it up into sections, so each section focuses just on one narrow topic with-in the bigger topic. So I will take the 400 page book and create a series of 8 books, each 50 pages long. But I don't publish them yet. Next I re-write and expand each of the little books, adding more information and details on each sub-topic, until the book is about 100 to 150 pages long. (For example: I may write a general book on homelessness, but then divide it into small books: one on how to survive being homeless during blizzards and hurricanes; one on how to build a shelter out of found items and how to maintain it for several years of homelessness; one on how to upgrade from a shanty tent to vandwelling; one on how to find safe access to food and water; one on the dangers you'll face while homeless and how to protect yourself; one on how to outfit a motorhome into a full time boondocking bugout machine; etc.)
I make my goal to have all my non-fiction books at least 100 pages and more then 100 pages if I have enough to say on the topic and usually I can get a book well over 150 pages.
In the end, I'll end up with 8 books of 100+ pages each (about 35,000+ words) that go into greater detail on various aspects of being homeless and surviving, instead of 1 big 400 page "general topic" book on homelessness in general, I brand them as a series, with matching covers, and NOW I do the free days.
When it comes to Free Days, I do the same thing for both my fiction and my non-fiction. I create a set of books, that has at least 5 volumes, with 10+ volumes prefered, and I will put volume 1 free, and volume 2 also free, and the following week, I put volumes 3 and 4 up for the .99c Countdown Deal.
What this does, is drive downloads to volumes 1 and 2, and gives readers a week to read 1 and 2, then come back with an interest in volumes 3 and for, which they find are on sale for .99c and so scoop them up. A week later, they have read 3 and 4, and come back looking for 5 and 6, and just buy them at full price because after reading the first 4 volumes, now they are hooked and want to read the rest of the set, and often will buy volumes 5, 6, 7, 8, 9, and 10 all at once.
I've found this to be the best method of using the Free Days to drive sales to your books.
I don't expect people to buy the first volumes, just because the free days boosted sales rank; instead I expect the people who downloaded the free books, to come back looking to buy the rest of the set.
And you can also only expect 1 sale to result for each 100 downloads. Free promos to paid sale conversion is only 10%, so if you got 100 downloads, you can expect 1 paid sale the following week. Any more then that is going to be a fluke. Say you got about 350 free downloads your first week, that will mean you can reasonably expect to see about 3 paid sales the following week.
With a series/serial/set of books, this multiples, to mean, you'll see those sales across all your books from the set. With slightly fewer sales of each of the final volumes.
In other words, had this been volume 1 of a 5 volume set, I would have seen something like this happen:
Volume 1 gets 350 free downloads.
Volume 2 gets 4 sales
Volume 3 gets 3 sales
Volume 4 gets 2 sales
Volume 5 gets 1 sale
This is why you see so many (KDP) authors who promote writing a series and then setting Volume 1 up as perma-free and then do free days on Volume 2 and Count down Deals on Volumes 3 and 4, never doing any free days or sales on final volumes. This is the method which has worked well or many authors, thus why it is recommended by so many of them. and it works in both fiction and non-fiction, because it is playing on, getting your reader hooked and coming back for more.
I like Select. I have books published all over the place, but the 170+ that I have on KDP are on Amazon exclusively, via Select and have been since 2010 - I was doing this method long before Kindle Unlimited came out, and I did not change my methods for KU. Because my books were already in Select, they got rolled over into KU, and now I borrows to consider. I'm not sure if KU is a longterm thing, so at the moment I'm not making plans around it.
But since KU came out that chart above, now looks something like this:
Volume 1 gets 350 free downloads.
Volume 2 gets 4 sales and 12 borrows
Volume 3 gets 3 sales and 9 borrows
Volume 4 gets 2 sales and 6 borrows
Volume 5 gets 1 sale and 3 borrows
I'm not sure what to think about that, other then, it's a dramatic increase of my income and, oddly, the addition of the borrows did not negatively impact the amount of sales I can expect after Free Days. And I see this trend in both my fiction and my non-fiction. Both do really well in KU. So, getting KU borrows is another aspect to consider with Select (you don't get the borrows if you're not in Select.)
Again, this is just what I've seen with my own books. I suspect genre plays a role here as well (I write Monster Porn, Bizarro, and Weird Tales, and my non-fiction is VERY autobiographical dealing with first person experience on topics I know extremely well: Homelessness; being an Adult with Autism; being transgendered; my culture and it's traditions; etc.), and I don't know if authors of other genres/topics have these same results with their books or not, so your results could be different.
