doasu.dev is a Fediverse instance that uses the ActivityPub protocol. In other words, users at this host can communicate with people that use software like Mastodon, Pleroma, Friendica, etc. all around the world.
This server runs the snac software and there is no automatic sign-up process.
I use #KOReader (https://koreader.rocks/) on my Kindle, and I like it far more than Kindle's native interface.
KOReader is also available on other platforms (e.g., Android, Linux) so you can try it there first!
It can even synchronise reading progress between devices, so I can resume reading on my tablet/phone. :)
(yes, I shill koreader. its awesome)