I wrote a very short book on Getting Things Done for hackers, which I also put up for sale on Amazon as a Kindle book, in addition to allowing people to get it for free from my website. As expected, it didn't sell much, perhaps a few dozen copies in total, but I got a small kick out of having a book for sale.
However, Amazon now wants me to set up some kind of US tax account, which is all fine and good and legal and appropriate, but it's not worth my time to do that. I've removed the book from Amazon, to keep my life simple.
It continues to be available on its own website, including in a Kindle format. For free. Under a CC-BY-SA 3.0 license. Since I see the download statistics for it, I know how many people download it. The PDF form is most popular, as it happens. All good.