Smashwords is an ebook vendor and distributor. They are unusual in that they offer ebooks in multiple formats-- Kindle, epub, even PDF. Every year in December, Smashwords runs a sale and offers authors the chance to opt in. This year the sale runs from Friday, December 18 through Friday, January 1, 2021. Thousands of Smashwords authors and publishers are providing readers deep discounts or even free books.
I have several books on Smashwords. The Sixth Disciple is always free; my fantasy novella Where Magic Rules is usually 99 but will be free during the sale, and No Safe Haven (the sequel to The Sixth Disciple ) The Nostalgia Gambit, King of Trees, and Turnabout will all be half price-- $1.49.
If you do download an ebook from Smashwords in Kindle's mobi format you can read it with the Kindle Cloud Reader or Kindle app, or you can to sideload it to a Kindle device via email. I have directions on this blog on how to do that,