Bringing an old Sony Xperia tablet to life as an eReader

I got gifted an old android 5.1 sony tablet from 2013 and to my amazement it can still install many apps from the Play Store and F-Droid, so I installed a manga reader called Kotatsu

Is there any subreddit dedicated to doing this sorta thing and keeping old devices running? Also whats a good eBook library/reader for my phone?