Here are my latest theories about offering your book for free on Amazon based on my own experiences. Keep in mind my book is science fiction and this won’t necessarily be the same for all genres. Also while writing this, my focus is on Amazon.
First of all, in order to get any exposure on Amazon you have to give away at least 2K books. The first time your book goes free you might giveaway that amount without getting any features. What may determine how many books you giveaway? I’ve found that the amount of reviews tends to impress both readers and promotional sites. For this reason, I emphasize having at least 10 reviews before going free or doing any promotion even if your book isn’t free. As for reviews, the only way to get them is to push for them. Offer free review copies on forums, book bloggers, etc, etc.
The second, third, fourth, etc, time your book goes free you won’t give away as many books unless you get a boost from one of the bigger e-book sites like E-reader News Today. I know because I’ve had some giveaways where I didn’t get a promo which resulted in only a few sales.
Most of these promotional sites are Amazon Affiliates. The rules have been changed for these affiliates. They aren’t allowed to offer near as many free books, so the odds of getting on these sites are limited. Your chances are better if you have more reviews because you are competing with other books to get on those kinds of sites. They look at the reviews and the average stars to determine who they will feature. So again…reviews are important.
Shortly after publishing a sequel to “Seer of Mars”, I managed to get a promo on a popular ebook site. So I set Seer of Mars to free for three days and gave away about 6,000 copies. I gained back three times the price of the ad. Copies of my first book are out there on Kindle devices everywhere with links on them that will lead to my second book on Amazon. (Always put links to your books in the beginning and ending of your Kindle books).
Lastly, a lot of people don’t read free books or they have so many it may take years for them to get to your book. However, I’ve had enough sales of the sequel to know that many people do read the free books.