Calibre. My brother used to use a Kindle to read but since stopped, and I wanted to get back into reading. Unfortunately, since it was a Kindle, I didn’t want it running on the web (y’know, Amazon). In order to be able to use it completely offline, I looked around and discovered the Calibre e-reader, which has a very user friendly way to send books to my Kindle.
Even as a standalone e-book reader app, it’s amazing, and I recommend it for anyone who wants to get back into reading.
You can find it @ GitHub - kovidgoyal/calibre: The official source code repository for the calibre ebook manager