I use calibre to manage epub/mobi books and move them onto my kindle. It is not an intuitive program.
Kindle Unlimited, has a fee, but is also a good source of books if you're more interested in genres rather than specific books. A lot of self-pub writers there.
Related to that, amazon sells those books for next to nothing. I got a few box sets totaling 18 books for the 3 dollars of digital credit I received by picking a longer shipping options on an order.
Of course, you can sail the seven seas and go the pirate route. Calibre will get them on your device.