NexusMods is working on their new post-vortex mod client, which does have Linux support. It can be found here on github.

There’s Librera FD, on F-droid. It’s the open source version of Librera on the google play store. I believe it strips out the google api/cloud stuff, but its db /reading status/etc are all easily synced via Nextcloud (which is what I do). The only real negative I’ve found with it is that it’s search for text in books is slow compared to more popular reading apps like Moon+ Reader.

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Aegis is for Android, Ravio is an iOS app.