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EPUB reader
(lemmy.world)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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Not FOSS, but Lithium on the play store has what you need and the app is free of tracking and ads.
I find open-source apps in this department to be still lacking.
It is not possible to make free open source app without tracking to save in the cloud.
"Book Reader" from f-droid has "sync folder" option, but I never tested it. I guess of you sync that folder with some other app (like syncthing, or google drive) you can have it in other places.