I usually download ebooks from Anna’s Archive and transfer it to my Kindle through the service. But 1/10 times it fails, with no apparent reason. The particular ebook in question is https://annas-archive.org/md5/e310a253a9d85992129cc2b8dd351556. What’s the way out?
Amazon added epub support to Kindles a while back, and deprecated mobi. It still has some issues with epub formatting though, I’ve had to tweak some files using a Calibre plugin to make them render properly on the kindle.