I use lithium. The free version has some limitations like not being able change the app to darkmode (although from what I remember the pages of the book itself can be changed to white/sepia/dark even in the free version). It has cross-device progress sync too, but I can’t remember if it was there in the free version). Worked well enough for me. Decided to get the pro version after a couple of years cause I had been using it a lot and didn’t have any gripes with the app. (Look for it in mobilism if you don’t want to pay for the pro version)