I can tell you that all my books are enrolled in the KDP Select program, which allows me to offer each book for free 5 days out of every 90-day period that it's enrolled. What good does giving away books do? A lot! It boosts the book ranking on Amazon, meaning more readers see it when they do a category search, such as "historical romance" or "western romance". This in turn leads to more sales. The major drawback is that you can only sell your ebooks on Amazon while they are in the program. It's a trade-off you should consider carefully. For me it has worked wonders.